Allow configurable client when server cannot take more requests

[trietsch]

As this driver's goal is to handle and send lots of requests, the default value for queued amount of requests (1024) is too low. This should be increased by using: httpClient.setMaxRequestsQueuedPerDestination(<int>);

[trietsch]

This is taken into account in release 0.2.1, however, the behavior of the client when the server cannot take more requests, should be configured by the end-user of the client.