Refactor PubSubHeaders, StompHeaders, MessageBuilder

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

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

MessageHolder holds the currently processed message in a ThreadLocal,
which allows PubSubMessageBuilder to automatically add a session id
to messages to be sent.
Add support for "system" STOMP session

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.
Add @MessageExceptionHandler

Similar to @ExceptionHandler but for message processing. Such a method
can send messages to both the message broker channel and the client
channel provided the client is subscribed to the target destination.