fix(pool): flush workItems after next tick to avoid dupe selection

In the event that a timeout occurs and a server instance is drained,
the work items were cleared before the server could signal it needed
to be cycled. By waiting until the next tick, we can ensure the
event makes it to the `Topology`, and the server is removed from
consideration for server selection.

NODE-2350

Author: mbroadst

lib/core/connection/connection.js

@@ -194,6 +194,20 @@ class Connection extends EventEmitter {
     this.socket.unref();
   }
 
+  /**
+   * Flush all work Items on this connection
+   *
+   * @param {*} err The error to propagate to the flushed work items
+   */
+  flush(err) {
+    while (this.workItems.length > 0) {
+      const workItem = this.workItems.shift();
+      if (workItem.cb) {
+        workItem.cb(err);
+      }
+    }
+  }
+
   /**
    * Destroy connection
    * @method

lib/core/connection/pool.js

@@ -251,20 +251,24 @@ function connectionFailureHandler(pool, event, err, conn) {
     // Remove the connection
     removeConnection(pool, conn);
 
-    // Flush all work Items on this connection
-    while (conn.workItems.length > 0) {
-      const workItem = conn.workItems.shift();
-      if (workItem.cb) workItem.cb(err);
-    }
-
-    if (pool.state !== DRAINING && pool.options.legacyCompatMode === false) {
+    if (
+      pool.state !== DRAINING &&
+      pool.state !== DESTROYED &&
+      pool.options.legacyCompatMode === false
+    ) {
       // since an error/close/timeout means pool invalidation in a
       // pre-CMAP world, we will issue a custom `drain` event here to
       // signal that the server should be recycled
       stateTransition(pool, DRAINING);
       pool.emit('drain', err);
+
+      // wait to flush work items so this server isn't selected again immediately
+      process.nextTick(() => conn.flush(err));
       return;
     }
+
+    // flush remaining work items
+    conn.flush(err);
   }
 
   // Did we catch a timeout, increment the numberOfConsecutiveTimeouts