Since external diffs are likely to be a bit slower than in-database
ones, add a mode that makes diffs external after they've been obsoleted
by events. This should strike a balance between performance and disk
space.

A background cron drives the majority of migrations, since diffs become
outdated through user actions.

The flag is on by default, but allows us to revert back
to the old behaviour if we encounter any problems.

Introduces the concept of Prerequisites for a CI build.
If a build has unmet prerequisites it will go through the
:preparing state before being made available to a runner.

There are no actual prerequisites yet, so current
behaviour is unchanged.

Hashed Storage rollback mechanism

See merge request gitlab-org/gitlab-ce!23955

Rollback is done similar to Migration for the Hashed Storage.
It also shares the same ExclusiveLease key to prevent both happening
at the same time.

All Hashed Storage related workers now share the same queue namespace
which allows for assigning dedicated workers easily.

Moved to HashedStorage namespace, and added them to the
`:hashed_storage` queue namespace