SIP - prevent race condition when writing contact header

The contact header field was stored in a member varialbe of Jami/SIP
accounts. There was a condition in which the contact header could
be concurrently accessed by multiple threads. This typically happens
when terminating calls in batch (when calling hangupCalls() for
instance).
Managment of SIP contact header in Jami and SIP accounts was
reworked to prevent such race.

Gitlab: #633

Change-Id: Ib9295070a5295969bf114ec29e66e36b1c5c5e03