res_pjsip_transport_management: Kill idle TCP connections.

res_pjsip_transport_management: Kill idle TCP connections.

"Idle" here means that someone connects to us and does not send a SIP
request. PJProject will not automatically time out such connections, so
it's up to Asterisk to do it instead.

When we receive an incoming TCP connection, we will start a timer
(equivalent to transaction timer D) waiting to receive an incoming
request. If we do not receive a request in that timeframe, then we will
shut down the TCP connection.

ASTERISK-25796 #close
Reported by George Joseph

AST-2016-005

Change-Id: I7b0d303e5d140d0ccaf2f7af562071e3d1130ac6