Close bucket store for users with no blocks synced

[harry671003]

What this PR does:

• If the tenant_cleanup_delay is large (say 24h), a deleted tenant can be continued to be synced by the store-gateways.
• This is because the s3://<bucket>/<tenantID>/markers/tenant-deletion-mark.json will be kept for 24h after the tenant has been deleted.
• If there are a lots of users being created and deleted in the cluster (canary testing), there could be a lot of deleted users synced by the store-gateway.
• One of the problems with having too many active users is the number of metrics that store-gateways will keep around. If there are 50K deleted users synced by a store-gateway and a total of 300 store-gateways in the cluster that'll be 15 million series.
• This change ensures that if no blocks are loaded for a tenant, their bucket stores can be closed immediately and the blocks_loaded metrics can be removed.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[harry671003]

This is no longer required if the tenant-delete-mark.json can be moved outside the tenant dir.
#5676