Issue 49300 - entryUSN is duplicated after memberOf operation
Bug Description: When we assign a member to a group we have two
oprations - group modification and user modification.
As a result, they both have the same entryUSN because USN Plugin
assigns entryUSN value in bepreop but increments the counter
in the postop and a lot of things can happen in between.
Fix Description: Increment the counter in bepreop together with
entryUSN assignment. Also, decrement the counter in bepostop if
the failuer has happened.
Add test suite to cover the change.
https://pagure.io/389-ds-base/issue/49300
Reviewed by: tbordaz (Thanks!)