[USER MODERATION] - 4th deliverable - lock moderation actions

[alejandromumo]

In this issue, we need to add a locking mechanism on user moderation actions (e.g. block, approve, restore) to avoid concurrent moderation actions on the same user.

The main concern about these requests is that we could end up with an inconsistent user state. For example:

• An admin blocks a user in the UI
• The admin immediately tries to restore the same user.

If we are not careful, the previous example could into a situation where the user appears to be restored (e.g. not blocked) but the records were not restored yet since they were being deleted when the second request was received.

See the discussion notes for more information on the issue