Moving the check out of the general requests, makes sure we don't have
any slowdown in the regular requests.
To keep the process performing this checks small, the check is still
performed inside a unicorn. But that is called from a process running
on the same server.
Because the checks are now done outside normal request, we can have a
simpler failure strategy:
The check is now performed in the background every
`circuitbreaker_check_interval`. Failures are logged in redis. The
failures are reset when the check succeeds. Per check we will try
`circuitbreaker_access_retries` times within
`circuitbreaker_storage_timeout` seconds.
When the number of failures exceeds
`circuitbreaker_failure_count_threshold`, we will block access to the
storage.
After `failure_reset_time` of no checks, we will clear the stored
failures. This could happen when the process that performs the checks
is not running.
The circuitbreaker now has 2 failure modes:
- Backing off: This will raise the `Gitlab::Git::Storage::Failing`
exception. Access to the shard is blocked temporarily.
- Circuit broken: This will raise the
`Gitlab::Git::Storage::CircuitBroken` exception. Access to the shard
will be blocked until the failures are reset.
