tapdb: add concrete implementation of supplycommit.CommitmentTracker and supplycommit.StateMachineStore

In this commit, we add concrete implementation of both CommitmentTracker
and StateMachineStore. This implements the persistence layer for the
supply commit state machine. The main method to understand is
ApplyStateTransition, as it implements the atomic update of all the
various components (tree, state transition, etc) on disk.

Author: Roasbeef