* Added DataBufferUtils.split variant that takes multiple delimiters
  as argument (instead of 1).
* Use this new split() variant from within StringDecoder, replacing
  its inefficient algorithm with the Knuth-Morris-Pratt algorithm.

org.junit.Assert.assertThat() is deprecated as of JUnit 4.13.

See gh-22894

Migrate all tests to consistently use BDD Mockito. Also add
checksyle rule to enforce going forwards.

Organize test imports to expand all '.*' static imports into
fully qualified imports.

This update will allow us to use additional checkstyle rules in
the future, and will also help if we migrate fully to AssertJ.

Delete the `Matches` class since it's no longer being used.

* Add limited checkstyles to test code

Add a limited set of checkstyle rules to the test codebase to improve
code consistency.

* Fix checksyle violations in test code

* Organize imports to fix checkstyle for test code

* Migrate to assertThatExceptionOfType

Migrate aware from ExpectedException rules to AssertJ exception
assertions. Also include a checkstyle rules to ensure that the
the ExpectedException is not accidentally used in the future.

See gh-22894

Includes fix for typo in visitor class names.

See gh-22909

Deprecate the public `StandardMetadata` constructors to make it clearer
that these classes should not be instantiated directly. A new
`AnnotationMetadata.introspect` factory method has been added which
can now be used to obtain instances.

This change will allow use to make the constructors package private
and drop the `nestedAnnotationsAsMap` parameter in a future release.

Closes gh-22906

Replace the existing ASM based readers with new implementations that
also support MergedAnnotations. The meta-data classes themselves are
now immutable, and constructed via separate reader classes.

The `SimpleMetadataReader` class has been updated to return the new
classes, however the old ones remain since some of them are public
and might be being used directly.

Closes gh-22884

Add `AnnotatedTypeMetaData.getAnnotations()` that can be used to access
annotation details using the `MergedAnnotations` interface.

Where possible, the existing annotation methods have been migrated to
call `getAnnotation()`, rather than needing their own implementation.

The existing ASM based meta-data implementations have not been updated
since they will be deprecated and replaced in a subsequent commit.

See gh-22884

Add some additional tests to provide more coverage of class and method
meta-data support.

See gh-22884

Update `StandardAnnotationMetadata` and `AnnotationMetadataReadingVisitor`
so that `java.lang.annotation` annotations are consistently skipped.

Closes gh-22885

Update ASM based metadata readers so that only RetentionPolicy.RUNTIME
annotations are exposed. This aligned behavior with the reflection based
implementation.

Closes gh-22886

Add a factory method to `MergedAnnotation` that allows an instance to
be created for an explicit collection of root annotations. This method
will allow ASM based readers to expose a `MergedAnnotation` instance
that has root annotations loaded from bytecode, and meta-annotations
loaded using reflection.

See gh-22884

Update TypeMappedAnnotation so that Strings can be used to represent
Class attribute values. This will allow ASM annotation readers to
present a `MergedAnnotation` instance without necessarily having the
actual class values on the classpath.

When the underlying value is a String, any calls to
`getValue(name, String.class)` or `asMap(Adapt.CLASS_TO_STRING)` will
simply return the original String. Calls that need the actual Class
result (such as `getClass`) will use `Class.forName` and may throw
a `ClassNotFoundException` at that point.

This commit also allows an empty Object[] to be used to represent
any empty primitive array.

See gh-22884

Rename `from` to `of` for the `MergedAnnotation` factory methods that
work with Maps. The previous name was a little confusing, especially
when an annotation source parameter was specified. The new method name
helps to make it clearer when the user is explicitly defining the
attributes of the annotation, as opposed to picking them up from the
source.

Add a `getTypeHierarchy()` method to `MergedAnnotation` that can be used
to return the full type hierarchy information. This method is
specifically designed to be used in combination with
`MergedAnnotationPredicates.unique`.

This update also allows us to delete the `parentAndType` method
from `AnnotatedElementUtils`.

Closes gh-22908

Closes gh-22912

Added two methods to DataBufferUtils:

* matcher(byte[]), which returns a Matcher object that can be used to
  find a delimiter in a data buffer.
* split(Publisher<DataBuffer>, byte[] delimiter), which splits a given
  stream of data buffers around a given delimiter.

Add method retainedSlice to the DataBuffer, defaulting to using
DataBufferUtils for retain, but allowing for ByteBuf specific override.

org.junit.Assert.assertThat() is deprecated as of JUnit 4.13.

See gh-22894

Ensure that results returned from keySet, entrySet & values are tracked
to remove case insensitive keys from the source map.

Closes gh-22821

Update `MergedAnnotation` with a `getRoot()` method that allows the
root direct annotation to be retrieved easily.

Closes gh-22818

See gh-22818

Following on 3ebbfa21 where the local
refCount was removed in favor of using the internal refCount of the
native data buffer, this commit ensures that LeakAwareDataBufferFactory
uses a PooledDataBufferFactory delegate by default.

There are also fixes for test issues with eager allocation uncovered by
these changes in StringDecoder and ResourceDecoder.

LeakAwareDataBuffer was keeping its own refCount rather than checking
through the delegate. This leads to false leak reports in a sequence
where an allocated buffer is retained and then sliced since it is not
aware of the changes to the refCount through the slice.

Due to the changes in a7425c81, we no
longer need to execute tests using the JMXMP protocol in PERFORMANCE
builds.

This commit removes the JMXMP constant from the TestGroup enum and
updates affected tests, thereby effectively including such tests in
the standard build from now on.

See gh-22757

Closes gh-22766

Add a convenience method that allows a `MergedAnnotation` to be
converted into an `AnnotationAttributes` instance. Also rename
the `MapValues` enum to `Adapt` which generally seems to read
better.

Closes gh-22738

Add tests for `MissingMergedAnnotation`

See gh-21697

Delete `AnnotationAttributeExtractor` which is package private and
no longer used.

Closes gh-22753

See gh-22726