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.