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
```

## What are the relevant issue numbers?

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

See merge request !6326