AGSMPLPUSH-52 "Remove synchronization in SockJsSession"

This PR contains three commits, but it is only 802e2f8b7848627178282b91e7942b5ff1f22376 that intended to be the single commit of this PR. This work builds upon https://github.com/aerogear/aerogear-simplepush-server/pull/74 and https://github.com/aerogear/aerogear-simplepush-server/pull/75 and once they have been processed this PR can be rebased.