fix(topology): enter STATE_CLOSING before draining waitQueue

NODE-2468

Author: mbroadst

lib/core/sdam/topology.js

@@ -312,14 +312,16 @@ class Topology extends EventEmitter {
     }
 
     options = options || {};
-    if (this.s.state === STATE_CLOSED) {
+    if (this.s.state === STATE_CLOSED || this.s.state === STATE_CLOSING) {
       if (typeof callback === 'function') {
         callback();
       }
 
       return;
     }
 
+    stateTransition(this, STATE_CLOSING);
+
     drainWaitQueue(this[kWaitQueue], new MongoError('Topology closed'));
     drainTimerQueue(this.s.connectionTimers);
 
@@ -336,8 +338,6 @@ class Topology extends EventEmitter {
       delete this.s.detectTopologyDescriptionChange;
     }
 
-    stateTransition(this, STATE_CLOSING);
-
     this.s.sessions.forEach(session => session.endSession());
     this.s.sessionPool.endAllPooledSessions(() => {
       eachAsync(