Define isLocalCommitTx and refactor contractcourt to use it

[ProofOfKeags]

Originating from this comment from @Roasbeef, we wish to have a minimal function that can reliably identify whether a broadcasted commitment transaction is ours or our peer's.

The suggested design is to create a pure function with the following type signature:

isLocalCommitTx :: ChannelType -> LocalChanCfg -> wire.MsgTx -> SignDescriptor -> bool

There is an open question about wether or not we need the fully formed SignDescriptor for this or just a piece of it. Once we have this function, we can improve the contractcourt code specifically in the commitSweepResolver