Issue 7578 - schema - attribute refcount is not maintained properly

Description:

The refcount mechanism for attributes is not correctly used, and this prevents
its use for synchronizing hashtable access. We increment the refcount, but we
don't decrement it correctly. This can potentially cause a rare race condition
with schema reload task and heap-use-after-free with searches running at the
same time as the reload task.

**CWE**: CWE-416 (Use After Free) via CWE-362 (Race Condition)

CI test assisted by: Cursor

relates: #7578

Reviewed by: progier, tbordaz, and spichugi(Thanks!!!)

Author: mreynolds389