Bug Description:
The hang condition:
- is not systematic
- occurs in rare case, for example here during the deletion of a replica.
- a thread is waiting for a dblock that an other thread "forgot" to
release.
- have always existed, at least since 1.4.0 but likely since 1.2.x

When deleting a replica, the replica is retrieved from
mapping tree structure (mtnode).
The replica is also retrieved through the mapping tree
when writing updates to the changelog.
When deleting the replica, mapping tree structure is cleared
after the changelog is deleted (that can take some cycles).
There is a window where an update can retrieve the replica,
from the not yet cleared MT, while the changelog being removed.
At the end, the update will update the changelog that is
currently removed and keeps an unfree lock in the DB.

Fix description:
Ideally mapping tree should be protected by a lock but it
is not done systematically (e.g. slapi_get_mapping_tree_node).
Using a lock looks an overkill and can probably introduce
deadlock and performance hit.
The idea of the fix is to reduce the window, moving the
mapping tree clear before the changelog removal.

https://pagure.io/389-ds-base/issue/50745

Reviewed by: ?