This moves tracking of the pushes since the last Git GC from PostgreSQL
to Redis. This reduces the number of writes on the "projects" table.
This in turn reduces the vacuuming overhead.
The lease used for incrementing the counter has been removed. This lease
was mostly put in place to prevent high database load but this isn't
needed anymore due to the counter now being stored in Redis.
Fixesgitlab-org/gitlab-ce#22125
Due to a stale NFS cache, it's possible that a branch lookup fails
while `git gc` is running and causes missing branches in merge requests.
Possible workaround for #15392
