Online GC: Lock contention on manifest deletes

Context

Found to be the cause for a severity3 production incident gitlab-com/gl-infra/production#7465 (closed).

Problem

During the investigation of the incident above, we detected many manifest deletions timing out on the DB server side due to a ShareLock contention. The root cause for this is still unknown and requires investigation.

See #732 (comment 1030488395) for more details and sample logs.