rpcserver: add REST workarounds, implement new RPC calls

This commit adds a few workarounds in order to concurrently support the
REST proxy as well as the regular gRPC interface. Additionally,
concrete support for the following RPC calls has been added:
GetTransactions, SubscriptTransactions, SubscribeInvoices, and
NewWitnessAddress.

server.go

@@ -48,7 +48,6 @@ type server struct {
 	bio      lnwallet.BlockChainIO
 	lnwallet *lnwallet.LightningWallet
 
-	// TODO(roasbeef): add to constructor
 	fundingMgr *fundingManager
 	chanDB     *channeldb.DB