lnd: add new preimageBeacon struct
lnd: add new preimageBeacon struct

In this commit, we add the preimageBeacon  which is an implementation
of both the contractcourt.WitnessBeacon and lnwallet.PreimageCache
interfaces. This will be used when closing channels to check to see if
the know the preimage, and also to communicate to any active contract
resolvers once we discover a preimage either on-chain or off-chain.

LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd