This commit also introduces a cache for attribute alias metadata.

Issue: SPR-11512

Issue: SPR-11512

Issue: SPR-13064

Issue: SPR-11512

Reinstate a variant of the `WebContentGenerator.checkAndPrepare` method
for projects that are using it.

This variant is now marked as deprecated.

Issue: SPR-11792

Issue: SPR-11512

This change ensures that the deprecated UserSessionRegistry is still
used if configured.

Before this commit the object that #this would refer to in
nested expressions within projection/selection clauses was always
the root context object. This was incorrect as it should be the
element being projected/selected over. This commit introduces
a scope root context object which is set upon entering a new
scope (like when entering a projection or selection). Any
object. With this change this kind of expression now behaves:

where #this is the element of list1. Unqualified references
are also resolved against this scope root context object.

Issues: SPR-10417, SPR-12035, SPR-13055

This commit introduces a test that will fail if SynthesizedAnnotation is
not public as is required by the contract for getProxyClass() in
java.lang.reflect.Proxy.

Issue: SPR-13057

Issue: SPR-13060

Issue: SPR-13056

Issue: SPR-12207

This commit introduces support for asynchronous return values thanks
to the new AsyncHandlerMethodReturnValueHandler interface. Out of
the box support for ListenableFuture is also provided.

Issue: SPR-12168

Includes an optimization for simple ByteBuffer duplication.

Issue: SPR-13056

Commit 3d86f15a added a lot of new feature in AbstractPropertyAccessor
shared by both DirectFieldAccessor and BeanWrapperImpl. This makes this
abstract class harder to implement for others having simpler use cases.

It turns that Spring Data has such use case; this commit split these new
features in a dedicated new base class, leaving AbstractPropertyAccessor
untouched.

Issue: SPR-12805

Enable public visibility on SynthetizedAnnotation to allow annotation
outside its package to be proxied properly. This commit is pending a
unit test that actually reproduces the problem.

Issue: SPR-13057

* SPR-11512:
  Support annotation attribute aliases and overrides via @AliasFor

This commit introduces first-class support for aliases for annotation
attributes. Specifically, this commit introduces a new @AliasFor
annotation that can be used to declare a pair of aliased attributes
within a single annotation or an alias from an attribute in a custom
composed annotation to an attribute in a meta-annotation.

To support @AliasFor within annotation instances, AnnotationUtils has
been overhauled to "synthesize" any annotations returned by "get" and
"find" searches. A SynthesizedAnnotation is an annotation that is
wrapped in a JDK dynamic proxy which provides run-time support for
@AliasFor semantics. SynthesizedAnnotationInvocationHandler is the
actual handler behind the proxy.

In addition, the contract for @AliasFor is fully validated, and an
AnnotationConfigurationException is thrown in case invalid
configuration is detected.

For example, @ContextConfiguration from the spring-test module is now
declared as follows:

    public @interface ContextConfiguration {

        @AliasFor(attribute = "locations")
        String[] value() default {};

        @AliasFor(attribute = "value")
        String[] locations() default {};

        // ...
    }

The following annotations and their related support classes have been
modified to use @AliasFor.

- @ManagedResource
- @ContextConfiguration
- @ActiveProfiles
- @TestExecutionListeners
- @TestPropertySource
- @Sql
- @ControllerAdvice
- @RequestMapping

Similarly, support for AnnotationAttributes has been reworked to
support @AliasFor as well. This allows for fine-grained control over
exactly which attributes are overridden within an annotation hierarchy.
In fact, it is now possible to declare an alias for the 'value'
attribute of a meta-annotation.

For example, given the revised declaration of @ContextConfiguration
above, one can now develop a composed annotation with a custom
attribute override as follows.

    @ContextConfiguration
    public @interface MyTestConfig {

        @AliasFor(
           annotation = ContextConfiguration.class,
           attribute = "locations"
        )
        String[] xmlFiles();

        // ...
    }

Consequently, the following are functionally equivalent.

- @MyTestConfig(xmlFiles = "test.xml")
- @ContextConfiguration("test.xml")
- @ContextConfiguration(locations = "test.xml").

Issue: SPR-11512, SPR-11513

Issue: SPR-13054