lnwallet/btcwallet: concretely implement the new TransactionSubscription interface
lnwallet/btcwallet: concretely implement the new TransactionSubscription interface

This commit returns the BtcWallet wrapper struct back to conformance to
the WalletController interface by adding support for
SubscribeTransactions.

The implementation of the lnwallet.TransactionSubscription consists
simply of a proxy goroutine which forwards notifications from the
wallet’s internal NotificaitonServer.

LightningNetwork/lnd,ddustin/lnd,aakselrod/lnd,ddustin/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd,AndrewSamokhvalov/lnd,LightningNetwork/lnd,bryanvu/lnd,bryanvu/lnd