contractcourt: add new ChannelArbitrator struct

In this commit, we add the primary struct of the package with a full
implementation. The duty of the ChannelArbitrator is to watch the set
of active contracts on a comment transaction and act accordingly if any
of their redemption criteria have been met. Potential criteria include:
an HTLC about to time out, and HTLC about to time out that we know the
preiamge to, or the remote party going to chain (forcing us to resolve
all pending contracts on chain).

The primary goroutine of this struct implements a persistent state
machine in order to ensure that mid contract resolution, we’re able to
properly survive restarts without losing our place, or forgetting about
a pending contract.

A ChannelArbitrator will stay alive until all contracts have been fully
resolved. This means that outside sub-systems no longer need to worry
about remembering to mark a channel as fully resolved, as it’s the job
of the ChannelArbitrator to do this task.

contractcourt/channel_arbitrator_test.go

@@ -0,0 +1 @@
+package contractcourt