Wait() option now accept *sync.WaitGroup

The original signature accept a boolean, and it feel like a little
verbose, since when people pass in this option, he/she always want to
pass a `true`.

Now if input `wg` is nil, it has same effect as passing `true` in
original code. Furthermore, if user want's finer grained control during
shutdown, one can pass in a predefined `wg`, so that server will wait
against it during shutdown.

server/options.go

@@ -2,6 +2,7 @@ package server
 
 import (
 	"context"
+	"sync"
 	"time"
 
 	"github.com/micro/go-micro/broker"
@@ -198,12 +199,18 @@ func WithRouter(r Router) Option {
 }
 
 // Wait tells the server to wait for requests to finish before exiting
-func Wait(b bool) Option {
+// If `wg` is nil, server only wait for completion of rpc handler.
+// For user need finer grained control, pass a concrete `wg` here, server will
+// wait against it on stop.
+func Wait(wg *sync.WaitGroup) Option {
 	return func(o *Options) {
 		if o.Context == nil {
 			o.Context = context.Background()
 		}
-		o.Context = context.WithValue(o.Context, "wait", b)
+		if wg == nil {
+			wg = new(sync.WaitGroup)
+		}
+		o.Context = context.WithValue(o.Context, "wait", wg)
 	}
 }