Graceful termination of artifacts server instances.
Graceful termination of artifacts server instances.

Summary:
+ Reduce downtime/possibility of errors while restarting artifacts store.
+ SIGTERM to running process will cause it to immediately close listening socket rejecting further
  requests (nginx is expected to immediately failover to the other slave). This will allow a new
  server to listen on the port immediately.
+ Any ongoing connections will continue to be serviced for 15 more seconds (configured in code)
  before they are terminated.

Test Plan: Run on Changes VM

Reviewers: jukka, kylec

Reviewed By: kylec

Subscribers: changesbot

Differential Revision: https://tails.corp.dropbox.com/D135272

dropbox/changes-artifacts,dropbox/changes-artifacts