sip: maintain SIP connections with contacts

This patch includes major changes on how messages are transmitted. It's the first
patch that uses the ConnectionManager. Each time a device is detected via the DHT,
Jami will ask a new p2p connection with that device and then will open a new SIP
channel. Also, the same mechanism exists when the user try to send a new message
through the DHT.

In the future, that SIP channel will be used to send vCards, presence status,
read status, etc.

Change-Id: I415553193accd8b1259ffdc037e684524fe03876