- "start" won't do anything if session is already encrypted with requested algorithm, and will raise a ConflictError if it's already encrypted but with an other algorithm
- implemented "stop", with an optional namespace to check we are stopping the expected algorithm
- "getSession" retrieve the current encryption session of a jid, if any
Plugin handling encryption can now register using host.registerEncryptionPlugin, and an encryption session can now be started using messageEncryptionStart bridge method.
This will make encryption handling more easy, as we now know if a session is clear or e2e encrypted, and which plugin handle it.
