Issue: SPR-10703

Issue: SRP-10868

Remove base class for STOMP-related message handler classes
(AbstractSimpMessageHandler), polish subclasses and fix issues with
more significant updates to STOMP broker relay.

Introduce base class for SubscribableChannel implementations providing
consistent logging for all channel implementations.

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.

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.

The "system" STOMP session is established at startup and can be used
to send messages without a client session, e.g. to support broadcasting
from a REST/HTTP handler method.

Rename to PubSubHeaderAccessor and StompHeaderAccessor
Move the renamed classes to support packages

Remove fromPayloadAndHeaders from MessageBuilder, just use
withPayload(..).copyHeaders(..) instead.

Import org.springframework.util.Assert rather than reactor.util.Assert

org.springframework.web.stomp is now
org.springframework.web.messaging.stomp

Also classes in the ~.stomp.server and ~.stomp.adapter packages have
been renamed.