Releases need to be made Atomic

[joneubank]

Issuing a release to an Alias where there were no indexes matching the release ID resulted in a 500 error. Before failing, Rollcall removed indexes from the alias before checking that the new indexes to replace with exists.

Need preflight checks to pass before updating alias.

[andricDu]

Additionally, we need to take into account the concurrency of multiple requests in flight. We should avoid two concurrent requests making modifications at the same time.

One possible way of doing this would be to use an AtomicReference holding the last run release/removal and using synchronized either on the method doing the work or possible on the AtomicReference itself.

[andricDu]

• [ ] Make a request for a release/remove thread safe
• [ ] Attempt to ensure success or failure before starting through a preflight check. #5
• [ ] Make the whole attempt atomic/transactional. All or nothing.