Issue: SPR-12730

Refine semantics of ID and TIMESTAMP headers provided to protected
MessageHeaders constructor.

Refactor internal implementation of MessageHeaderAccessor.

Support mutating headers from a single thread while a message is being
built (e.g. StompDecoder creating message + then adding session id).

Improve immutablity in NativeMessageHeaderAccessor and in
StompHeaderAccessor.

Optimize object creation for initializing messages and subsequent
accessing their headers.

Introduce MessageHeaderAccessorFactory support to enable applying a
common strategies for ID and TIMESTAMP generation to every message.

Add MessageBuilder shortcut factory method for creating messages from
payload and a full-prepared MessageHeaders instance. Also add
equivalent constructors to GenericMessage and ErrorMessage.

Issue: SPR-11468

Following discussion with Gary.

Issue: SPR-11468

Allow a subclass to modify MessageHeaders as well as override the
strategy for or even skip having `ID` and `TIMESTAMP` headers.

Issue: SPR-11468

Moved messaging.support.converter to messaging.converter; moved messaging.support.tcp to messaging.tcp; moved messaging.support.channel to messaging.support itself; moved handler.condition to handler itself; reworked handler.method into handler.support

Apply consistent styling to new classes introduced in Spring 4.0.

- Javadoc line wrapping, whitespace and formatting
- General code whitespace
- Consistent Assert.notNull messages

This change adds an alternative UUID generation strategy to use by
default in MessageHeaders. Instead of using SecureRandom for each
new UUID, SecureRandom is used only for the initial seed to be
provided java.util.Random. Thereafter the same Random instance is
used instead. This provides improved performance while id's are
still random but less securely so.

After this change, annotated message handling methods configured to use
a destination prefix (e.g. "/app") no longer have to include the prefix
in their mapping. For example if a client sends a message to "/app/foo"
the annotated methods should be mapped with @MessageMapping("/foo").

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

Minor formatting and Javadoc changes.

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.