Per-channel mutexes

[TheBlueMatt]

We should move channels into their own mutexes in ChannelManager, both for performance, and to make reasoning about things like #398 easier. I think the correct way is to have per-channel pending_msg_events queue and then have the cross-channel events (forward_htlcs and claimable_htlcs) live in their own mutexes. While we're at it, we should index channel ids by peer pubkeys, fixing #105.

[ariard]

See slack discussion on high-throughput : https://lightningdevkit.slack.com/archives/CUBTAU938/p1590731602032400

[TheBlueMatt]

Note that if/when we do this we'll probably need to add per-ChannelMonitor locks in ChainMonitor as otherwise we'll just end up bottlenecked on Persister update time.

[TheBlueMatt]

We did this!