Consolidates new, messaging-related classes from spring-context and
spring-websocket into one module.

The SubscriptionRegistry and implementations are now in a package
together with SimpleBrokerWebMessageHandler and primarily support
with matching subscriptions to messages. Subscriptions can contain
patterns as supported by AntPathMatcher.

StopmWebSocketHandler no longer keeps track of subscriptions and simply
ignores messages without a subscription id, since it has no way of
knowing broker-specific destination semantics for patterns.

Add a PublishSubscribeChannel implementation of SubscribableChannel.

Minor formatting and Javadoc changes.

SimpleBrokerWebMessageHandler can be used as an alternative to the
StompRelayWebMessageHandler.

* SPR-10694:
  ConversionService JSR-356 Encoder/Decoder adapters
  Add ConversionService support for ByteBuffers

Develop support class allowing JSR-356 Encoder and Decoder interfaces
to delegate to Spring's ConversionService.

Issue: SPR-10694

Add ByteBufferConverter that is registered by default with the
DefaultConversionService. Allows conversion from/to a ByteBuffer and
byte[] or to any type that can be converted via a byte[].

Issue: SPR-10712

When headers are being removed based on pattern matching, both the
new header names and the original header names need to be matched
against the pattern. Previously, only new headers were being
considered resulting in any matching original headers not being
removed.

When a header is being set, verify that the type that's provided is
legal for the header that's been set. For example, the error channel's
type must be a MessageChannel or a String.

The method that performs type verification is protected so that it
can be overriden by sub-classes. It is expected that an overriding
method will call the super method.

Revert changes to DefaultDocumentLoader accidentally committed in
60532cbd.

Polish MethodReference code formatting and remove duplicate code.

Update the cached MethodExecutor in MethodReference to include the
target type. Prevents the incorrect use of the cache when the
SpEL expression refers to a different target object.

Issue: SPR-10657

Remove the extra 's' from the class name

The Reactor Environment (that's used by the TcpClient) manages a
number of threads. To ensure that these threads are cleaned up
Environment.shutdown() must be called when the Environment is no
longer needed.

Minor polish to formatting and assertion messages.

Issue: SPR-10587

Effectively rename HttpHeaders.getIfNotModifiedSince() to
getIfModifiedSince() by adding a new method and deprecating the
old one.

Issue: SPR-10600

Refine the SQL statements contained in exceptions thrown from batch
updates based on BatchUpdateException.getUpdateCounts().

Issue: SPR-10677

Update the reference guide jdbcTemplate update example to include the
column name.

Issue: SPR-10625

Ensure any exception from calls to batchUpdate, on databases that
support batch operations, contains all the SQL statements. Prior to this
commit only the last SQL statement would be returned with the exception.

Issue: SPR-10677

A new type MessageHeaderAccesssor provides read/write access to
MessageHeaders along with typed getter/setter methods along the lines
of the existing MessageBuilder methods (internally MessageBuilder
merely delegates to MessageHeaderAccessor). This class is extensible
with sub-classes expected to provide typed getter/setter methods for
specific categories of message headers.

NativeMessageHeaderAccessor is one specific sub-class that further
provides read/write access to headers from some external message
source (e.g. STOMP headers). Native headers are stored in a separate
MultiValueMap and kept under a specific key.