This commit is a step to split the lnwallet package. It puts the Input
interface and implementations in a separate package along with all their
dependencies from lnwallet.
In this commit, we add a new file: briefcase.go. The contents of this
file are the ArbitratorLog. This log will be used by the internal state
machine of each Channel Arbitrator to ensure that each state transition
is fully reflected on-disk, to ensure that the state machine is durable
and able to survive restarts.
This commit also adds a new implementation of the ArbitratorLog
interface backed by boltdb.
