This commit fixes an issue where DataBufferUtils.join() would not
release databuffers that preceded an error signal.

Issue: SPR-17025

Issue: SPR-17198

Issue: SPR-17188

Issue: SPR-16946

Issue: SPR-16968

Ensure that `this.` is used consistently when accessing class
fields.

Issue: SPR-16968

Fix checkstyle violations for javadoc.

Issue: SPR-16968

Issue: SPR-16833

Issue: SPR-16838

(cherry picked from commit a2765c00)

Issue: SPR-16832

Includes nullable return value for getClassLoader() with corresponding notes in applicable javadoc.

Issue: SPR-16866

Resource.isReadable() is defined to semantically imply exists() now.

Issue: SPR-16832

Issue: SPR-16646

Includes consistent use of ClassUtils.toClassArray (as non-null variant)

Issue: SPR-16523

This commit introduces DataBuffer.asInputStream(boolean), that can
release the underlying buffer when the stream is closed.

Furthermore, this commit adds additional javadoc.

Issue: SPR-16444

Issue: SPR-16337

Issue: SPR-16365

Fixed by creating `Callable`-based variants, as explained in the JIRA
issue.

Issue: SPR-16403

Use DataBufferUtils.compose instead of writeAggregator to combine
multiple data buffers into one, as the write aggregator would not work
when the initial data buffer did not have enough capacity to contain
all subsequent buffers.

Removed writeAggregator, as it is no longer needed.

Issue: SPR-16365

Added a utility method that composes data buffers into a single buffer.
Depending on the `DataBuffer` implementation, the returned buffer may be
a single buffer containing all data of the provided buffers, or it may
be a true composite that contains references to the buffers.

Issue: SPR-16365

As a consequence of dropping CompositeByteBuf (see prior commit),
DataBuffers fluxes that are aggregated with Flux.reduce(BiFunction) are
now required to be released, as the composite no longer holds a
reference to subsequent data buffers.

For this purpose, DataBufferUtils now has a writeAggregator that can be
used with Flux.reduce, and that released the subsequent buffers
properly.

Issue: SPR-16351

Prior to this commit, NettyDataBuffer had a optimization in
write(ByteBuf...), where it used a CompositeByteBuf to hold the original
and the parameter buffer.
Unfortunately, this procedure has nasty consequences when splicing
buffers (see https://stackoverflow.com/a/48111196/839733).

As of this commit, NettyDataBuffer stopped using CompositeByteBuf, and
 simply does ByteBuf.write().

Issue: SPR-16351

Issue: SPR-16334

Issue: SPR-16293

Issue: SPR-16156

This commit uses the existing CompositeByteBuf if present, as opposed
to creating a new composite for every call to
NettyDataBuffer.write(ByteBuf...)

Issue: SPR-16180

Issue: SPR-16140

Includes fixes for invalid exception declarations in Mockito-based unit tests. Also includes FreeMarker 2.3.27, Commons Pool 2.4.3, JSON-P 1.1.2.

Issue: SPR-16157

Issue: SPR-16146