Issue: SPR-17086

Issue: SPR-17267
Issue: SPR-17372

Issue: SPR-17464

Issue: SPR-17467

Issue: SPR-17464

1. Avoid re-creating the List with delimited byte arrays on every
request if using the default delimiters which don't vary by charset.

2. Replace flatMap with flatMapIterable for splitOnDelimiter.

3. Avoid going through DataBufferUtils#join, and unnecessarily creating
Flux from the List, since the join method needs a list anyway.

Issue: SPR-17408

Fix ResourceRegionEncoder so that it checks for resource existance
before writing boundaries. Also defer data buffer allocation until
necessary.

Issue: SPR-17419

This commit fixes a memory leak that occurs when reading from a
AsynchronousFileChannel, and cancelling the subscription.

Issue: SPR-17419

Issue: SPR-17267

Issue: SPR-17418

Issue: SPR-17413

Issue: SPR-17417

Issue: SPR-17408

Issue: SPR-17397

As discussed in KT-25165, from a Kotlin POV enum constructors
have no parameter, this is an "implementation detail"
required for running on the JVM, so it seems relevant to skip
Kotlin reflection in that case and just delegate to Java
reflection.

Issue: SPR-16931

Issue: SPR-17371

Issue: SPR-17347

Issue: SPR-17333

Includes use of Files.getLastModifiedTime for NIO Paths, preservation of NIO-based resolution on createRelative, deprecation of PathResource, and consistent use of getContentLengthLong over getContentLength.

Issue: SPR-17320

Issue: SPR-17267

This was a package private class in spring-messaging since 5.0, and was
recently made public in 5.1. This commit promotes it to spring-core
where it belongs next to all other ListenableFuture support classes.

Follow-up refactoring for SPR-17336

Previously, both nullsLow(Comparator<T>) and
nullsHigh(Comparator<T>) returned a
NullSafeComparator which treated nulls as being
high. This commit corrects this typo.

Prefix the opening brace with a single space in some classes.
Replace tab character with space in javadoc of AttributeAccessor.
Also fix some other trivial formatting errors.

Closes gh-1979

Issue: SPR-17246

Issue: SPR-17282

Issue: SPR-17267

1. Helper method to eliminate duplication in formatting (de-)serialized
   values for logging introduced with prior commit #e62298.

2. Helper method for TRACE vs DEBUG logging with different details.

Issue: SPR-17254

Issue: SPR-17272

At DEBUG show up to 100 chars, at TRACE show full formatted value.

Note that the formatValue helper method is duplicated a number of times
in this commit. A utility method will likely be added in spring-core
through an extra commit.

Issue: SPR-17254

Includes fallback match for collection/map dependency if qualified.

Issue: SPR-17272
Issue: SPR-17197

Issue: SPR-17267