Move MessageChannel fields up to AbstractMessageBroker
Detect unsent DISCONNECT messages

This change uses a ChannelInterceptor (inserted at index 0) to detect
when a DISCONNECT message is precluded from being sent on the
clientInboundChannel. This can happen if another interceptor allows
a runtime exception out from preSend or returns false.

It is crucial for such messages to be processed, so when detected
they're processed still.

Issue: SPR-12218