lnwallet: add FeeEstimator interface, StaticFeeEstimator implementation

This commit adds the FeeEstimator interface, which can be used for
future fee calculation implementations. Currently, there is only the
StaticFeeEstimator implementation, which returns the same fee rate for
any transaction.

rpcserver.go

@@ -688,7 +688,7 @@ func (r *rpcServer) fetchActiveChannel(chanPoint wire.OutPoint) (*lnwallet.Light
 	// Otherwise, we create a fully populated channel state machine which
 	// uses the db channel as backing storage.
 	return lnwallet.NewLightningChannel(r.server.lnwallet.Signer, nil,
-		dbChan)
+		lnwallet.StaticFeeEstimator{FeeRate: 250}, dbChan)
 }
 
 // forceCloseChan executes a unilateral close of the target channel by