Improve unit test coverage of the client pushback mechanism

Improve the unit test coverage of the HBASE-5162 client pushback mechanism.

Currently we have:

• TestClientExponentialBackoff: Sanity checking of client backoff policy implementations.
• TestClientPushback: An medium test that checks if the response to a submitted put returns region load data and verifies the client tracks the load data correctly.

We don't have an end to end test that closes the loop between server feedback and changes in client behavior.