Assure ResponseHandler is called once and only once

Change executeAsync to return a ListenableFuture instead of a CheckedFuture since checked future requires multiple calls to response handler
Remove use of CheckedFuture from most APIs
Properly track server state in ExponentialBackOff
Align api of Duration with DataSize

Record DataSize unit so unit does not change with printing
Fixes for review feedback
Add constructor with environment and pool to TestingNodeModule
Add TestingJmxModule