fix(sdam): don't emit close every time a child server closes

The `close` event was erroneously emitted every time a child server
closed, which was not consistent with the legacy topology's
behavior. Instead, the event is now emitted when the topology itself
is closed.

NODE-2251

lib/core/sdam/topology.js

@@ -26,6 +26,7 @@ const resolveClusterTime = require('../topologies/shared').resolveClusterTime;
 const SrvPoller = require('./srv_polling').SrvPoller;
 const getMMAPError = require('../topologies/shared').getMMAPError;
 const makeStateMachine = require('../utils').makeStateMachine;
+const eachAsync = require('../utils').eachAsync;
 
 // Global state
 let globalTopologyCounter = 0;
@@ -360,31 +361,22 @@ class Topology extends EventEmitter {
     // defer state transition because we may need to send an `endSessions` command above
     stateTransition(this, STATE_CLOSING);
 
-    const servers = this.s.servers;
-    if (servers.size === 0) {
-      stateTransition(this, STATE_CLOSED);
-      if (typeof callback === 'function') {
-        callback(null, null);
-      }
+    eachAsync(
+      Array.from(this.s.servers.values()),
+      (server, cb) => destroyServer(server, this, options, cb),
+      () => {
+        this.s.servers.clear();
 
-      return;
-    }
+        // emit an event for close
+        this.emit('topologyClosed', new monitoring.TopologyClosedEvent(this.s.id));
 
-    // destroy all child servers
-    let destroyed = 0;
-    servers.forEach(server =>
-      destroyServer(server, this, options, () => {
-        destroyed++;
-        if (destroyed === servers.size) {
-          // emit an event for close
-          this.emit('topologyClosed', new monitoring.TopologyClosedEvent(this.s.id));
-
-          stateTransition(this, STATE_CLOSED);
-          if (typeof callback === 'function') {
-            callback(null, null);
-          }
+        stateTransition(this, STATE_CLOSED);
+        this.emit('close');
+
+        if (typeof callback === 'function') {
+          callback();
         }
-      })
+      }
     );
   }
 
@@ -942,7 +934,6 @@ function createAndConnectServer(topology, serverDescription) {
   server.once('connect', serverConnectEventHandler(server, topology));
   server.on('descriptionReceived', topology.serverUpdateHandler.bind(topology));
   server.on('error', serverErrorEventHandler(server, topology));
-  server.on('close', () => topology.emit('close', server));
   server.connect();
   return server;
 }