call: wait for CONNECTED state to send messages

In multi-device, we buffer outgoing messages and send them to subcalls
when their SIP session become available.
For this purpose we waited for CallState::ACTIVE which is not enough,
since ConnectionState should be CONNECTED to ensure the SIp session to
be availble.

Change-Id: I2d84fdd4b4340e9ba198f75530212e83be575920
Tuleap: #1154