Update thread pool doco

Update the Play doco to discuss how thread pools should be configured for database access:

http://www.playframework.com/documentation/2.1.3/ThreadPools

Perhaps we can make this config even easier via Akka config.

Given jdbc, thread pools should be size to the # of connections available to a db pool. The queue len given a thread pool executor should be size to the # of connections to the db pool * the max number of concurrent requests per action.