[LockManager] Generallized wiki field to a domain field.

* In some cases (like certain qualified file backends), a domain
  may be desired that is not simple a wiki ID.
* Also added a LockManangerGroup::config() function to export
  the configuration of a lock manager.

Change-Id: I5a75c55be3cc9017ed8a717d6d30e466fd00340f