User directory synchronisation can hang when synchronising a user with updated details and new memberships

Steps to reproduce:

1. Set up Confluence with a transactional database like PostgreSQL
2. Add any kind of synchronising directory to Confluence (LDAP, Crowd)
3. Synchronise Confluence with the directory
4. Edit a user in the directory server: Change first name and add the user to a new group
5. Synchronise Confluence with the directory

Outcome:
Synchronisation will never complete due to a database deadlock.