feat(pool): add support for resetting the connection pool

SDAM requires us to be able to rest the connection pool in certain
conditions, but we have traditionally not done this in the node
driver. This implements support for the reset, and unblocks us from
implementing tests like "cursors survive primary stepdown"

NODE-1682

Author: mbroadst

lib/core/connection/pool.js

@@ -26,6 +26,8 @@ var CONNECTED = 'connected';
 var DESTROYING = 'destroying';
 var DESTROYED = 'destroyed';
 
+const CONNECTION_EVENTS = ['error', 'close', 'timeout', 'parseError', 'connect', 'message'];
+
 var _id = 0;
 
 /**
@@ -631,9 +633,6 @@ Pool.prototype.unref = function() {
   });
 };
 
-// Events
-var events = ['error', 'close', 'timeout', 'parseError', 'connect', 'message'];
-
 // Destroy the connections
 function destroy(self, connections, options, callback) {
   let connectionCount = connections.length;
@@ -669,10 +668,7 @@ function destroy(self, connections, options, callback) {
 
   // Destroy all connections
   connections.forEach(conn => {
-    for (var i = 0; i < events.length; i++) {
-      conn.removeAllListeners(events[i]);
-    }
-
+    CONNECTION_EVENTS.forEach(eventName => conn.removeAllListeners(eventName));
     conn.destroy(options, connectionDestroyed);
   });
 }
@@ -758,19 +754,44 @@ Pool.prototype.destroy = function(force, callback) {
  * @param {function} [callback]
  */
 Pool.prototype.reset = function(callback) {
-  // this.destroy(true, err => {
-  //   if (err && typeof callback === 'function') {
-  //     callback(err, null);
-  //     return;
-  //   }
+  const connections = this.availableConnections.concat(this.inUseConnections);
+  let connectionCount = connections.length;
+  const connectionDestroyed = () => {
+    connectionCount--;
+    if (connectionCount > 0) {
+      return;
+    }
 
-  //   stateTransition(this, DISCONNECTED);
-  //   this.connect();
+    // clear all pool state
+    this.inUseConnections = [];
+    this.availableConnections = [];
+    this.connectingConnections = 0;
+    this.executing = false;
+    this.reconnectConnection = null;
+    this.numberOfConsecutiveTimeouts = 0;
+    this.connectionIndex = 0;
+    this.retriesLeft = this.options.reconnectTries;
+    this.reconnectId = null;
+
+    // create an initial connection, and kick off execution again
+    _createConnection(this);
 
-  //   if (typeof callback === 'function') callback(null, null);
-  // });
+    if (typeof callback === 'function') {
+      callback(null, null);
+    }
+  };
+
+  // if we already have no connections, just reset state and callback
+  if (connectionCount === 0) {
+    connectionDestroyed();
+    return;
+  }
 
-  if (typeof callback === 'function') callback();
+  // destroy all connections
+  connections.forEach(conn => {
+    CONNECTION_EVENTS.forEach(eventName => conn.removeAllListeners(eventName));
+    conn.destroy({ force: true }, connectionDestroyed);
+  });
 };
 
 // Prepare the buffer that Pool.prototype.write() uses to send to the server