media_decoder: refactor locking model

In SIPCall, requestKeyFrame & setVideoOrientation are using
pjsip on the main thread. To avoid this, use the io thread as
it's io operations that should not be done on main thread.

Moreover, some operations from recorder were called from pjsip
but not retriggered, causing jami->pjsip->jami operations
called with lock that could be called from pjsip nor jami causing
potential deadlocks.

Finally, audioDecoder_/receiveThread_ should be protected in
AudioRtpSession.

This fixes crashes for ut_conference

GitLab: #900
GitLab: #901
GitLab: #902
Change-Id: I055b297d25bc417e685f4c31d6105e4f6dab52e6