possibility to clean-up delegated directories

Issue Summary

Delegated directories are not pruned/clean up automatically. If the user is removed from the LDAP the entry won't be deleted from Crowd's database.

There is a directory pruning plugin for Crowd itself, but there is nothing for other products using Embedded Crowd. The idea is to bring some automatic cleanup for all products.

Steps to Reproduce

1. Create delegated directory pointing at LDAP
2. User from delegated directory logs in
3. Remove user from LDAP

Expected Results

User is removed from Crowd.

Actual Results

User is not removed from Crowd.

Workaround

For Crowd itself - use plugin (https://marketplace.atlassian.com/apps/1218630/delegated-directory-pruning-for-crowd?hosting=server&tab=overview).

For products using Embedded Crowd:

Currently the only option is to not use delegated directories and use cached directories instead.