Sessions are never cleared during LDAP user directory sync

Summary

Confluence does not clear its session during a full sync, making Hibernate track a gigantic pool of objects, i.e. synchronizing a large LDAP is virtually impossible; since it takes days to complete (even with very good hardware).
With more than 60K users the synchronization can take up to 5 days. But with flushing session and clearing it, the synchronization can be completed in 10 minutes.

Environment

• All Confluence versions with crowd-api-2.8.3
• All supported databases are impacted

Steps to Reproduce

1. Install Confluence 5.8 and set up connection to large LDAP instance
2. Perform an upgrade to 5.10

Expected Results

Confluence is able to finish the synchronization in couple hours

Actual Results

The synchronization is taking up to 6 days

Workaround

No verified workaround available.
bjarne.holen843903303 proposed to use session.flush(), session.clear() via. the HibernateTemplate after each user was synchronized.