call management: add ringing timeout

Currently there is no timeout for received calls. If a peer is
connected but doesn't answer a call, then this call will stay in
RINGING state 'forever' until the calling user manually aborts
it or peer becomes unreachable.

This patch implements a timeout system: When a call starts ringing
a job is scheduled in a fixed amount of time (corresponding to a new
user preference, by default 30s) to check whether the call is still
ringing or not. If the call is still ringing after timeout, hangup()
is called with 486 (BUSY) state and ringing stops.

Change-Id: I49218f5d4bdef8e4a8cee35fa6dde80f572b46c6
Gitlab: #42