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

Author: daprahamian

lib/topologies/mongos.js

@@ -574,7 +574,7 @@ function reconnectProxies(self, proxies, callback) {
           // 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

@@ -1051,6 +1051,7 @@ Server.prototype.destroy = function(options) {
 
   // Destroy the pool
   this.s.pool.destroy(options.force);
+  this.s.pool = undefined;
 };
 
 /**