fix: connection leak if wait queue member cancelled (#2563)

A connection could potentially be leaked if a wait queue member was cancelled (due
to timeout) before execution. In these cases we should return the connection back to
the list of connections for future use or pruning.

NODE-2865

Author: jswangjunsheng

src/cmap/connection_pool.ts

@@ -565,18 +565,14 @@ function processWaitQueue(pool: ConnectionPool) {
   if (pool.waitQueueSize && (maxPoolSize <= 0 || pool.totalConnectionCount < maxPoolSize)) {
     createConnection(pool, (err, connection) => {
       const waitQueueMember = pool[kWaitQueue].shift();
-      if (!waitQueueMember) {
+      if (!waitQueueMember || waitQueueMember[kCancelled]) {
         if (!err && connection) {
           pool[kConnections].push(connection);
         }
 
         return;
       }
 
-      if (waitQueueMember[kCancelled]) {
-        return;
-      }
-
       if (err) {
         pool.emit(
           ConnectionPool.CONNECTION_CHECK_OUT_FAILED,