fix backchannel setup to use a custom pauseStream()

[nschimme]

This fixes the Illegal Instruction trace seen on Frigate 0.16 beta:

#0  0x008ac564 in typeinfo for destRecord ()
#1  0x005f00ac in StreamState::pause (this=0x7599b420) at OnDemandServerMediaSubsession.cpp:611
#2  0x005ee238 in OnDemandServerMediaSubsession::pauseStream (this=0x77b106a0, streamToken=0x7599b420) at OnDemandServerMediaSubsession.cpp:269
#3  0x005c8fa4 in RTSPServer::RTSPClientSession::handleCmd_SETUP_afterLookup2 (this=0x75992a80, sms=0x75d53110) at RTSPServer.cpp:1512
#4  0x005c8858 in RTSPServer::RTSPClientSession::handleCmd_SETUP_afterLookup1 (this=0x75992a80, sms=0x75d53110) at RTSPServer.cpp:1404
#5  0x005c87e0 in RTSPServer::RTSPClientSession::SETUPLookupCompletionFunction1 (clientData=0x75992a80, sessionLookedUp=0x75d53110) at RTSPServer.cpp:1397
#6  0x005beef0 in GenericMediaServer::lookupServerMediaSession (this=0x75a070a0, streamName=0x759c0304 "ch0", completionFunc=0x5c879c <RTSPServer::RTSPClientSession::SETUPLookupCompletionFunction1(void*, ServerMediaSession*)>, completionClientData=0x75992a80) at GenericMediaServer.cpp:48
#7  0x005c877c in RTSPServer::RTSPClientSession::handleCmd_SETUP (this=0x75992a80, ourClientConnection=0x75965020, urlPreSuffix=0x759c0304 "ch0", urlSuffix=0x759c03cc "track3", fullRequestStr=0x759650ac "SETUP rtsp://192.168.0.46/ch0/track3 RTSP/1.0\r\nTransport: RTP/AVP/TCP;unicast;interleaved=4-5\r\nCSeq: 8\r\nAuthorization: Digest username=\"thingino\", realm=\"LIVE555 Streaming Media\", nonce=\"5cbdfe5d3e493"...) at RTSPServer.cpp:1390
#8  0x005c5d6c in RTSPServer::RTSPClientConnection::handleRequestBytes (this=0x75965020, newBytesRead=326) at RTSPServer.cpp:890
#9  0x005c08f0 in GenericMediaServer::ClientConnection::incomingRequestHandler (this=0x75965020) at GenericMediaServer.cpp:323
#10 0x005c06f8 in GenericMediaServer::ClientConnection::incomingRequestHandler (instance=0x75965020) at GenericMediaServer.cpp:304
#11 0x00645968 in BasicTaskScheduler::SingleStep (this=0x75b95650, maxDelayTime=0) at BasicTaskScheduler.cpp:171
#12 0x00649e50 in BasicTaskScheduler0::doEventLoop (this=0x75b95650, watchVariable=0x8b0064 <global_rtsp_thread_signal>) at BasicTaskScheduler0.cpp:87
#13 0x004d3114 in RTSP::start (this=0x8bf2f0 <rtsp>) at src/RTSP.cpp:151
#14 0x004d37d0 in RTSP::run (arg=0x8bf2f0 <rtsp>) at src/RTSP.cpp:185
#15 0x007cbd20 in start ()
#16 0x007da7ec in __clone ()

[nschimme]

Confirmed working by upgrading all of my cameras and using stock 0.16 Frigate