Add StompCodec

Previously, the broker relay's TCP client used Reactor's built in
delimited codec as part of its parsing of STOMP frames. \0 was used as
the delimiter. This worked for most STOMP frames but, crucially,
not for frames with a body that contained \0: when such a frame was
received it would be truncated.

This commit adds a custom codec that parses STOMP frames more
intelligently. It honours the content-length header allowing it to
correctly parse frames with a body that contains \0. The codec largely
delegates to two new classes: StompEncoder and StompDecoder. For
consistency, code that previously used StompMessageConverter has been
reworked to use these new encoder and decoder classes.

Issue: SPR-10818
Remove CONNECT-related message buffer from STOMP relay

Before this change, the StompProtocolHandler always responded to
clients with a CONNECTED frame, while the STOMP broker relay
independantly forwarded the client CONNECT to the broker and waited
for the CONNECTED frame back. That meant the relay had to buffer
client messages until it received the CONNECTED response from
the message broker.

This change ensures that clients wait for a CONNECTED frame from
the message broker. The broker relay forwards the CONNECT frame to
the broker. The broker responds with a CONNECTED frame, which the
relay then forwards to the client. As a result, a (well-written)
client will not send any messages to the relay until the connection
to the broker is fully established.

The StompProtcolHandler can now be configured whether to send CONNECTED
frame back. By default that is off. So when using the simple broker,
the StompProtocolHandler can still respond with CONNECTED frames.

The relay's handling of a connection being dropped has also been
improved. When a connection for a client relay session is dropped
an ERROR frame will be sent back to the client. If a connection is
closed as part of a DISCONNECT frame being sent, no ERROR frame
is sent back to the client. When the connection for the system relay
session is dropped, an event is published indicating that the broker
is unavailable. Reactor's TcpClient will then attempt to re-restablish
the connection.
Add heart-beat support to STOMP broker relay

Previously, the STOMP broker relay did not support heart-beats. It sent
0,0 in the heart-beats header for its own CONNECTED message, and set the
heart-beats header to 0,0 when it was forwarding a CONNECTED from from a
client to the broker.

The broker relay now supports heart-beats for the system relay session.
It will send heart-beats at the send interval that's been negotiated
with the broker and will also expect to receive heart-beats at the
receive interval that's been negotiated with the broker. The receive
interval is multiplied by a factor of three to satisfy the STOMP spec's
suggestion of lenience and ActiveMQ 5.8.0's heart-beat behaviour (see
AMQ-4710).

The broker relay also supports heart-beats between clients and the
broker. For any given client's relay session, any heart-beats received
from the client are forwarded on to the broker and any heart-beats
received from the broker are sent back to the client.

Internally, a heart-beat is represented as a Message with a byte array
payload containing the single byte of new line ('\n') character and
'empty' headers. SubscriptionMethodReturnValueHandler has been updated
to default the message type to SimpMessageType.MESSAGE. This eases
the distinction between a heartbeat and a message that's been created
from a return value from application code.
Upgrade to Reactor 1.0.0.M3
Improve handling of missed heartbeats

Previously, when a broker heartbeat was mnissed, the STOMP connection
would be left in a semi-disconnected state such that, for example, the
read and write idle callbacks would still be active, even though
the underlying TCP connection had been nulled out.

As part of disconnecting the STOMP connection, this commit closes the
underlying TCP connection when a heartbeat's missed which cancels the
read and write idle callbacks. It also now copes with the underlying
TCP connection being null when sending a heartbeat to the broker. This
protects again a race condition between the write idle callback being
fired, such that a heartbeat needs to be sent, and the connection
being nulled out due to it being closed.
Polish
Merge multiple pull requests from wilkinsona

* broker-relay:
  Polish
  Improve handling of missed heartbeats
  Upgrade to Reactor 1.0.0.M3
  Add heart-beat support to STOMP broker relay
  Remove CONNECT-related message buffer from STOMP relay
  Add StompCodec