The following has been done within multiple iterations:

Refactored URL parsing and removed unnecessary doc in local helper.
Refactored to use AtomicReference instead of singleton Map.
Adjusted tests of paging to test that pages contain expected objects.
Added check and test for deleting non-empy zone.
Removed RrsetWapper and used tailMap in listing.
Added @VisibleForTesting annotations.
Added fails to expected exceptions. Added error message checks.
Addressed codacy suggestions.
Assigned project ID to the helper and test.
Added pool of executors instead of spawning one threads.
Reduced number of threads.