Move pushes_since_gc to Redis

What does this MR do?

This moves tracking of the pushes since the last Git GC to Redis to reduce DB load.

Are there points in the code the reviewer needs to double check?

Styling mostly.

Why was this MR needed?

Updating this column can lead to a lot of writes which in turn puts a lot of load on table vacuuming. For example, in the last hour alone we had 5067 UPDATEs for this column (per InfluxDB):

> select count(increment_pushes_since_gc_call_count) from sidekiq_transactions where time > now() - 1h;
name: sidekiq_transactions
--------------------------
time                    count
1473780996567714622     5067

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/22125

cc @stanhu @pcarranza