Fixed locking in RequestHandlers and documented that RequestHandlers are called without locking (#9945)

* Added SynchronizedRequestHandler which locks the session for the whole request
* Made request handlers which do not do do heavy IO implement SynchronizedRequestHandler
* Fixed locking for GlobalResourceHandler, ConnectorResourceHandler, PublishedFileHandler

Change-Id: I0e7b442a9392828f1930685f194dc4f898d0f525