Cosmetics for COMETD-481 (Advice "multiple-clients" may be generated due to race condition).
 COMETD-482 - Implement hook for transport failures.

 Implementation for BayeuxClient.
Guarding against NPE when calling close() if the connection has not been opened.
 COMETD-482 - Implement hook for transport failures.

 Implementation for the JavaScript client.
More tests on forwarding behaviour.
Tests for COMETD-484.
Fixes COMETD-488 (NullpointerException while another Oort Node is connecting that uses OortObjects).
Merged branch '2.7.x' into 'master'.