fix(cmap): don't run min connection thread if no minimum specified

NODE-2459

Author: millerick

lib/cmap/connection_pool.js

@@ -362,7 +362,7 @@ class ConnectionPool extends EventEmitter {
 }
 
 function ensureMinPoolSize(pool) {
-  if (pool.closed) {
+  if (pool.closed || pool.options.minPoolSize === 0) {
     return;
   }