Put the "migrate when renaming" part of the Hashed Storage under a feature flag

As part of the rollout strategy in GitLab dot Com and as a way to get confidence in Hashed Storage and move it to GA &75 (closed) we should be able to disable the "rename" part introduced in : https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19747. See discussion here: https://gitlab.com/gitlab-com/infrastructure/issues/4772.

As I believe it doesn't make sense to isolate the configuration into two separate, and also because I believe it's better to have it on by default when Hashed Storage is enabled, I propose we have a feature flag to disable (as opposing to disable the behavior and having the feature flag to enable it). So it can behave as a kill-switch in order to do some timed rollout testing both behavior in isolation.

cc @vsizov @stanhu @ash.mckenzie @akaemmerle @dbalexandre