Docserver: Make ObjectStore set calls return Futures

This allows (but does not require) consumers to wait on
the completion of Set/SetMulti operations.

BUG=None
R=kalman@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/658733004

Cr-Commit-Position: refs/heads/master@{#300938}
6 files changed