fix(mongos): fix connection leak when mongos reconnects

Our mongos reconnect logic involved creating a new Server+Pool
per proxy per iteration of the reconnect logic. These instances
were never cleaned up, resulting in a huge number of connections
when the proxies finally reconnected.

Fixes NODE-1403

lib/topologies/mongos.js

@@ -568,13 +568,13 @@ function reconnectProxies(self, proxies, callback) {
           _self.on('parseError', handleEvent(self, 'parseError'));
 
           // Move to the connected servers
-          moveServerFrom(self.disconnectedProxies, self.connectedProxies, _self);
+          moveServerFrom(self.connectingProxies, self.connectedProxies, _self);
           // Emit topology Change
           emitTopologyDescriptionChanged(self);
           // Emit joined event
           self.emit('joined', 'mongos', _self);
         });
-      } else if (event === 'connect' && self.authenticating) {
+      } else {
         // Move from connectingProxies
         moveServerFrom(self.connectingProxies, self.disconnectedProxies, _self);
         this.destroy();
@@ -613,6 +613,9 @@ function reconnectProxies(self, proxies, callback) {
         })
       );
 
+      _server.destroy();
+      removeProxyFrom(self.disconnectedProxies, _server);
+
       // Relay the server description change
       server.on('serverDescriptionChanged', function(event) {
         self.emit('serverDescriptionChanged', event);
@@ -635,6 +638,7 @@ function reconnectProxies(self, proxies, callback) {
       relayEvents(server, self, ['commandStarted', 'commandSucceeded', 'commandFailed']);
 
       // Connect to proxy
+      self.connectingProxies.push(server);
       server.connect(self.s.connectOptions);
     }, i);
   }

lib/topologies/server.js

@@ -1001,6 +1001,8 @@ var listeners = ['close', 'error', 'timeout', 'parseError', 'connect'];
  * @param {boolean} [options.force=false] Force destroy the pool
  */
 Server.prototype.destroy = function(options) {
+  if (this._destroyed) return;
+
   options = options || {};
   var self = this;
 
@@ -1013,7 +1015,10 @@ Server.prototype.destroy = function(options) {
   }
 
   // No pool, return
-  if (!self.s.pool) return;
+  if (!self.s.pool) {
+    this._destroyed = true;
+    return;
+  }
 
   // Emit close event
   if (options.emitClose) {
@@ -1048,6 +1053,7 @@ Server.prototype.destroy = function(options) {
 
   // Destroy the pool
   this.s.pool.destroy(options.force);
+  this._destroyed = true;
 };
 
 /**