data transfer: fix data transfer destruction

fix crash occuring during data transfer destruction due to nullptr access
when the turn transport is destroyed.

list of changes:
* fix Turn/PJSIP callbacks registration (was a NPA due to unique_ptr dtor implementation)
* add a shutdown method to GenericSocket.
* implement shutdown into TlsSession and TurnTransport.
* use shutdown to unlock PeerConnection blocked into a read.
* do not call blocking methods that raises CtrlMsg into the eventloop.
* destroy clients/servers before turn socket .
* fix Turn pool cache destruction causing bad mem free.

Change-Id: I9995ce9419e8af965def9328a5de4a8973a4334f
Reviewed-by: Anthony Léonard <anthony.leonard@savoirfairelinux.com>