Every 5 minutes, check to see if the docs volumes have been updated,
and if so, release them. This means we can serve the docs volumes
from replicated read-only volumes with only a 5 minute delay.
Since this does not coordinate with the docs publishing jobs, we
may end up releasing partial updates, however, those jobs, since they
use rsync, should tolerate this.
Change-Id: I082ae6f37af9a6e12ad62b0cc4cb45e631a0935b