Serialize access to XMPP server
Serialize access to XMPP server

XMPP expects one stanza to be sent at a time.  Before this commit, two
incoming SMS, arriving at the same time, could trigger two XML stanzas
to be sent at the same time.  They could collide on the wire and
corrupt each other.

The mutex serializes access to the XMPP server so that only one stanza
is sent at a time.

I considered serializing access with a channel but that required more
architectural overhead than a simple mutex.

mndrix/sms-over-xmpp