Fixes COMETD-427 (Improve BayeuxServer.createIfAbsent()).

Introduced method:

MarkedReference<ServerChannel> BayeuxServer.createChannelIfAbsent(String, Initializer...)

that returns the channel and a flag telling whether the channel was
created or it already existed.

Deprecated BayeuxServer.createIfAbsent(String, Initializer...) and
updated all usages to the new method.
Merged branch '2.7.x' into 'master'.
Merged branch '2.7.x' into 'master'.