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
4f6ad25b0d