QPID-4534: unify client heartbeat system properties/connection url options.

* Connection url 'heartbeat' broker-option (and deprecated 'idle_timeout') now understood for all protocols
* System property 'qpid.heartbeat' (and deprecated 'amqj.heartbeat.delay' and 'idle_timeout') now understood for all protocols
* Enhanced heartbeat system tests
* Docbook updates

Original patch from Keith Wall, plus updates from Robbie Gemmell

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1537313 13f79535-47bb-0310-9956-ffa450edef68