Implement concurrent request handling in SprayServer

Summary:
Currently the Spray server was serving only one reuest at a time
as there is just one actor handling all the requests.
This change wraps every rest endpoint method in a Future block,
which allows for execution of each block by multiple threads.
The server also uses its own separate Execution Context.

Test Plan: Manual testing done, cluster_config test passes, UI testing partially done, Acceptance Test partially done.

Reviewers: jd, nitro, kt, j

Reviewed By: kt

Differential Revision: http://phabricator.scale-data.com/D2551