Made StackObjectPool's maxIdle parameter configurable in RedisClientPool

[felipehummel]

The commit message says it all, but trying to explain better:

I was using RedisClientPool in a environment with 24 threads heavily using Redis (and doing other stuff). The problem is that, as maxIdle of StackObjectPool defaults to 8, when more than 8 objects were borrowed and then returned to the pool, this maxIdle threshold was hit. As ~24 objects are borrowed but pool only maintains up to 8 idle objects. That means that every time an object is returned and there is more than 8 idle objects in the pool, the bottom object in the stack pool is destroyed. Then causing a gigantic destroy-create-destroy-create cycle. As the number of Sockets being opened and closed to Redis increased, I guess, it also exhausted the ephemeral port range giving the error:

java.lang.RuntimeException: java.net.NoRouteToHostException: Cannot assign requested address

Changing the maxIdle parameter to 30 stopped all the problems. I've also check in the Redis log that a huge number of connections were being dropped and accepted again and again and again.

[felipehummel]

An alternative is to do something like:

class CustomRedisClientPool(host: String, port: Int, maxIdle: Int = 8) extends RedisClientPool(host, port) {
  override val pool = new StackObjectPool(new RedisClientFactory(host, port), maxIdle)
}

One problem is that RedisClientFactory is private[redis]