Set mongoOptions.server.reconnectTries = Infinity.

Setting mongoOptions.server.auto_reconnect was removed by #7880 via commit
0ffb9ac824, though it seems the Server
options still respect autoReconnect, even in version 2.2 of the driver:
http://mongodb.github.io/node-mongodb-native/2.2/api/Server.html

That said, having inspected the code of the `mongodb` package, I do not
believe this change is really critical, since the default value for
autoReconnect appears to be true.

More importantly, I can't find any support in the code of the `mongodb`
npm package or its dependencies for the claim that setting
mongoOptions.server.reconnectTries to 0 is the same as making it infinite,
so this commit sets it to Infinity.

packages/mongo/mongo_driver.js

@@ -131,14 +131,17 @@ MongoConnection = function (url, options) {
   self._onFailoverHook = new Hook;
 
   var mongoOptions = _.extend({
-      db: { safe: true },
-      // Set reconnectTries to 0 which means keep trying to reconnect forever,
-      // rather than the default of losing the connection permanently after 30
-      // retries (separated by 1000ms).
-      server: { reconnectTries: 0 },
-      replSet: {}
+    db: { safe: true },
+    // http://mongodb.github.io/node-mongodb-native/2.2/api/Server.html
+    server: {
+      // Reconnect on error.
+      autoReconnect: true,
+      // Try to reconnect forever, instead of stopping after 30 tries (the
+      // default), with each attempt separated by 1000ms.
+      reconnectTries: Infinity
     },
-    Mongo._connectionOptions);
+    replSet: {}
+  }, Mongo._connectionOptions);
 
   // Disable the native parser by default, unless specifically enabled
   // in the mongo URL.