Connection based heartbeat.

AMQP offers a heartbeat feature to ensure that the application layer
promptly finds out about disrupted connections (and also completely
unresponsive peers). If the client requests heartbeats on connection, rabbit
server will regularly send messages to each connections with the expectation of
a response.
To acheive this, each driver connection object spawn a thread that
send/retrieve heartbeat packets exchanged between the server and the
client.
To protect the concurrency access to the kombu connection between the
driver and this thread use a lock that always prioritize the
heartbeat thread. So when the heartbeat thread wakes up it will acquire the
lock quickly, to ensure we have no heartbeat starvation when the driver
sends a lot of messages.
Also when we are polling the broker, the lock can be held for a long
time by the 'consume' method, so this one does the heartbeat stuffs itself.