fix(replset): don't leak servers failing to connect

The change to include all connecting servers in the local
`connectingServers` array lead to a "server leak" for nodes that
we are not able to connect to. Now we track them, and only add
new ones on a successful reconnect attempt.

NODE-2270

lib/core/topologies/replset.js

@@ -277,15 +277,26 @@ function rexecuteOperations(self) {
 }
 
 function connectNewServers(self, servers, callback) {
+  // No new servers
+  if (servers.length === 0) {
+    return callback();
+  }
+
   // Count lefts
   var count = servers.length;
   var error = null;
 
+  function done() {
+    count = count - 1;
+    if (count === 0) {
+      callback(error);
+    }
+  }
+
   // Handle events
   var _handleEvent = function(self, event) {
     return function(err) {
       var _self = this;
-      count = count - 1;
 
       // Destroyed
       if (self.state === DESTROYED || self.state === UNREFERENCED) {
@@ -332,17 +343,10 @@ function connectNewServers(self, servers, callback) {
 
       // Rexecute any stalled operation
       rexecuteOperations(self);
-
-      // Are we done finish up callback
-      if (count === 0) {
-        callback(error);
-      }
+      done();
     };
   };
 
-  // No new servers
-  if (count === 0) return callback();
-
   // Execute method
   function execute(_server, i) {
     setTimeout(function() {
@@ -351,6 +355,18 @@ function connectNewServers(self, servers, callback) {
         return;
       }
 
+      // remove existing connecting server if it's failed to connect, otherwise
+      // wait for that server to connect
+      const existingServerIdx = self.s.connectingServers.findIndex(s => s.name === _server);
+      if (existingServerIdx >= 0) {
+        const connectingServer = self.s.connectingServers[existingServerIdx];
+        connectingServer.destroy({ force: true });
+
+        self.s.connectingServers.splice(existingServerIdx, 1);
+        done();
+        return;
+      }
+
       // Create a new server instance
       var server = new Server(
         Object.assign({}, self.s.options, {