In this commit, we add a new method: ProcessContractResolution. This
will be used by entities of the contract court package to notify us
whenever they discover that we can resolve an incoming contract
off-chain after the outgoing contract was fully resolved on-chain.
We’ll take a contractcourt.ResolutionMsg and map it to the proper
internal package so we can fully resolve an active circuit.
In this commit, we add a new abstraction to the package: the mailBox.
The mailBox is a non-blocking, concurrent safe, in-order queue for
delivering messages to a given channelLink instance. With this
abstraction in place, we can now allow the switch to no longer launch a
new goroutine for each forwarded HTLC, or instantiated user payment.
