Issue: SPR-17374

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.

Prior to this commit, no `ResourceUrlProvider` was configured
in WebFlux (no bean was contributed by the WebFlux infrastructure).
Also, several `ResourceTransformer` instances that extend the
`ResourceTransformerSupport` base class need a `ResourceUrlProvider`
to resolve absolute URLs when rewriting resource URLs. At this point,
no `ResourceUrlProvider` was configured and they could only resolve
relative URLs.

This commit contributes a new `ResourceUrlProvider` to the WebFlux
configuration; this bean can be reused by the WebFlux infrastructure and
application code.

This also automatically configure this shared `ResourceUrlProvider`
instance on the resource chain where needed.

Issue: SPR-17433

Issue: SPR-17419

Issue: SPR-17409

Issue: SPR-17409

Issue: SPR-17430

Prior to this commit, `ResourceTransformerSupport.toAbsolutePath`
would call `StringUtils.applyRelativePath` in all cases. But this
implementation is prepending the given path even if the relative path
starts with `"/"`.

This commit skips the entire operation if the given path is absolute,
i.e. it starts with `"/"`.

Issue: SPR-17432

Issue: SPR-17419

Issue: SPR-17419

This commit fixes a memory leak in ServerSentEventHttpMessageWriter
that occurs when the input stream contains an error. Test added as well.

Issue: SPR-17419

Prior to this commit, the `ResourceUrlEncodingFilter` would wrap the
response and keep a reference to the request. When
`HttpServletResponse.encodeURL` is later called during view rendering,
the filter looks at the request and extracts context mapping information
in order to resolve resource paths in views.

This approach is flawed, when the filter is used with JSPs - if the
request is forwarded to the container by the `InternalResourceView`,
the request information is overwritten by the container. When the view
is being rendered, the information available in the request is outdated
and does not allow to correctly compute that context mapping
information.

This commit ensures that that information is being extracted from the
request as soon as the `ResourceUrlProvider` is set as a request
attribute.

Issue: SPR-17421

Issue: SPR-17405

Issue: SPR-17408

This commit fixes a memory leak in Jaxb2XmlEncoder that occurs when
the input stream contains an error. Test added as well.

Issue: SPR-17419

Issue: SPR-17419

The flatMap operation in writeAndFlushWith could buffer internally.

Issue: SPR-17424

Issue: SPR-17400

Towards Californium SR2 to confirm fix for SPR-17306.

Issue: SPR-16983

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-17420

Issue: SPR-17372

Issue: SPR-17267

The fix for SPR-17178 switched from debug to warn level warning for
all sub-classes of AbstractHandlerExceptionResolver where the request
concerned the DefaultHandlerExceptionResolver only.

This commit restores the original DEBUG level logging that was in
AbstractHandlerExceptionResolver from before SPR-17178. In addition
DefaultHandlerExceptionResolver registers a warnLogCategory by default
which enables warn logging and hence fulfilling the original goal
for SPR-17178.

Issue: SPR-17383

This reverts commit 162112.

Instead we'll go for a bigger rollback of the change from SPR-17178 so
that only DefaultHandlerExceptionResolver does warn logging by default.

Issue: SPR-17388