Merge pull request #7 from DoNewsCode/Reasno-patch-1

The original default shutdown event is tightly coupled to HTTP and gRPC servers. If one of any servers is missing, goroutines will be leaked. This PR removes the
default shutdown events. These events can be added by using the WithShutdownEvents option.

BREAKING CHANGE

pool.go

@@ -47,7 +47,6 @@ import (
 	"sync"
 	"time"
 
-	"github.com/DoNewsCode/core"
 	"github.com/DoNewsCode/core/contract"
 	"github.com/DoNewsCode/core/events"
 	"github.com/oklog/run"
@@ -66,7 +65,7 @@ func NewPool(options ...ProviderOptionFunc) func(contract.Dispatcher) *Pool {
 			concurrency:    10,
 			timeout:        10 * time.Second,
 			dispatcher:     dispatcher,
-			shutdownEvents: []interface{}{core.OnHTTPServerShutdown, core.OnGRPCServerShutdown},
+			shutdownEvents: []interface{}{},
 		}
 		for _, f := range options {
 			f(&pool)