TranscodeSub + Direct Connections

[j0sh]

Tracking issue for the proposal outlined in https://github.com/livepeer/go-livepeer-basicnet/issues/21#issuecomment-369310870

• [x] On-chain tracking of new jobs and assignments in transcoder.
• [x] Transcoder prepares to receive a direct connection (set up listeners if needed)
• [x] Transcoder sends TranscoderSub(T_MultiAddr, T_NodeID, StrmID, T_Sig) if assigned a job
• [x] Broadcaster handling for TranscoderSub request ; establish a direct connection
• [x] Transcoder handling of direct connection; SUB to broadcaster
• [x] Broadcaster handling of incoming SUB requests via the direct connection
• [ ] Verify TranscodeSub messages on broadcaster. Requires https://github.com/livepeer/go-livepeer-basicnet/issues/31 or monitoring the event log.
• [x] relay fallback (eg, if transcoder is NAT'd)
• [x] retransmissions for broadcaster misses
• [x] disconnect handling