Refine BrokerAvailabilityEvent behavior

Add accessor for brokerAvailable in AbstractBrokerMessageHandler
Ensure brokerAvailable is set even if eventPublisher is not
Add tests BrokerMessageHandlerTests

Turn off brokerAvailable when StompBrokerRelayMessageHandler stops
Actually stop message handling when brokerAvailable=false
Improve log messages

Issue: SPR-11563
Refine Reactor-based TCP client implementation

Configure explicitly use of SynchronousDispatcher instead of the one
used otherwise by default (RingBufferDispatcher). As a result TCP
optations are now scoped to Netty's threads.

Remove Environment field. It is no longer required to shut it down
since we're now using SynchronousDispatcher by default.

Replace connection.in() with connection.consume() when composing
connection handling. The former creates a Stream for further composing,
e.g. via map(), filter() but all we need is to read a message.

Provide additional constructor that aceepts a pre-configured Reactor
TcpClient instance.

Issue: SPR-11531
Rename ReactorNettyTcpClient to ReactorTcpClient

Even though Netty is a required dependency of reactor-tcp at present,
there is no hard dependency in the spring-messaging Reactor-based
implementation.