Add a config switch to toggle the deletion logic in background verifier
The background verifier verifies that the replicas that should exist according to the metadata actually exist. If they don't, the verifier would delete the metadata record and Praefect would no longer consider such a replica to exist.
This process has a race due repository renames not being atomic. During rename operations, the replicas are first moved on the disk and then the corresponding update is made to the metadata. If the verifier sees the in-between state where the replica is already moved on the disk but the metadata hasn't yet been updated, it would incorrectly consider a replica as non-existent and delete its metadata record.
This problem will ultimately be fixed by !4101 (merged) which makes the renames atomic. Until then, we should keep the background verifier from deleting any metadata records as this could cause Praefect to lose replicas. !4101 (merged) is blocked on the Rails tests not exercising production code paths of Praefect sufficiently.
In order to enable the verifier and test it before hand, let's add a config option that allows to disable the deletion logic itself but lets the verifier run. If the deletion logic is disabled, the verifier would mark the replica as successfully verified so the work can proceed but would still produce accurate metrics in Prometheus and logs on the number of invalid metadata records.