multi: replace per channel sigPool with global daemon level sigPool

In this commit, we remove the per channel `sigPool` within the
`lnwallet.LightningChannel` struct. With this change, we ensure that as
the number of channels grows, the number of gouroutines idling in the
sigPool stays constant. It's the case that currently on the daemon, most
channels are likely inactive, with only a hand full actually
consistently carrying out channel updates. As a result, this change
should reduce the amount of idle CPU usage, as we have less active
goroutines in select loops.

In order to make this change, the `SigPool` itself has been publicly
exported such that outside callers can make a `SigPool` and pass it into
newly created channels. Since the sig pool now lives outside the
channel, we were also able to do away with the Stop() method on the
channel all together.

Finally, the server is the sub-system that is currently responsible for
managing the `SigPool` within lnd.

rpcserver.go

@@ -1431,7 +1431,6 @@ func (r *rpcServer) CloseChannel(in *lnrpc.CloseChannelRequest,
 	if err != nil {
 		return err
 	}
-	channel.Stop()
 
 	// If a force closure was requested, then we'll handle all the details
 	// around the creation and broadcast of the unilateral closure
@@ -1667,7 +1666,7 @@ func (r *rpcServer) fetchOpenDbChannel(chanPoint wire.OutPoint) (
 
 // fetchActiveChannel attempts to locate a channel identified by its channel
 // point from the database's set of all currently opened channels and
-// return it as a fully popuplated state machine
+// return it as a fully populated state machine
 func (r *rpcServer) fetchActiveChannel(chanPoint wire.OutPoint) (
 	*lnwallet.LightningChannel, error) {
 
@@ -1680,7 +1679,7 @@ func (r *rpcServer) fetchActiveChannel(chanPoint wire.OutPoint) (
 	// we create a fully populated channel state machine which
 	// uses the db channel as backing storage.
 	return lnwallet.NewLightningChannel(
-		r.server.cc.wallet.Cfg.Signer, nil, dbChan,
+		r.server.cc.wallet.Cfg.Signer, nil, dbChan, nil,
 	)
 }