WIP: TranscodeSub

[j0sh]

Implements https://github.com/livepeer/go-livepeer-basicnet/issues/29

Notes:

• While the external API is a TranscodeSub method on VideoNetwork. This comes rather late in the commit series. I should squash the earlier commits for a cleaner history. https://github.com/livepeer/go-livepeer-basicnet/pull/32/commits/242fee8cd3601bdddb87888a1cb553814fcedfb5
• We reuse the libp2p listener that the transcoder already has open for the direct connection
• Transcoders handle the direct connection via implementing Notifiee on its own subscriber. https://github.com/livepeer/go-livepeer-basicnet/pull/32/commits/8da37d4f63899abd887e2ef2e511dd336f9c3c7e
• Updates libp2p in order to work around an issue with the peerstore that was affecting tests.