add option poolReturnToHead which is passed through to genericPool

add option `poolReturnToHead` to facilitate reduction of pool size
after demand recedes. As the current round robin check now keeps on
touching all the connections and never lets them expire under decent
traffic.

Author: tarun

lib/defaults.js

@@ -33,6 +33,9 @@ var defaults = module.exports = {
   //frequeny to check for idle clients within the client pool
   reapIntervalMillis: 1000,
 
+  // re-use most recent connections, allowing older connections to be reaped.
+  poolReturnToHead: false,
+
   //pool log function / boolean
   poolLog: false,
 

lib/pool.js

@@ -20,6 +20,7 @@ var pools = {
       max: clientConfig.poolSize || defaults.poolSize,
       idleTimeoutMillis: clientConfig.poolIdleTimeout || defaults.poolIdleTimeout,
       reapIntervalMillis: clientConfig.reapIntervalMillis || defaults.reapIntervalMillis,
+      returnToHead: clientConfig.poolReturnToHead || defaults.poolReturnToHead,
       log: clientConfig.poolLog || defaults.poolLog,
       create: function(cb) {
         var client = new pools.Client(clientConfig);