gddo-server: simplify background tasks with time.Tick

These tasks operate concurrently with the request flow anyway, so each
task should be on its own tick loop.

Change-Id: I129212b297db15395f7852e5b73e66081c63fbd0
Reviewed-on: https://go-review.googlesource.com/66652
Reviewed-by: Tuo Shan <shantuo@google.com>
2 files changed