Better connection management (#LEMONLDAP-663, #LEMONLDAP-664):

- Create userDBFinish, passwordDBFinish and registerDBFinish methods
- Call finish methods for each backend in process
- Call LDAP unbind only when necessary
- Unbind if error in RegisterDBLDAP

git-svn-id: svn://svn.forge.objectweb.org/svnroot/lemonldap/branches/lemonldap-ng_version_1_4-bugfixes@3427 1dbb9719-a921-0410-b57f-c3a383c2c641