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.