All calls to connection.send should go through the wrapper in order to
prevent breaking the connection by calling it in the not connected
state.

This prevents from accessing Strophe internals directly and allows to
block 'send' calls while disconnected. The latter breaks the stream
resume logic by putting stanzas onto the send queue.

TODO make jitsi strophe plugins use the XmppConnection wrapper whenever
possible instead of interacting with the Strophe.Connection directly.