ITS#1986 cache add failed

[hyc@highlandsun.com]

It looks like this can happen if bdb_cache_entry_db_lock() fails to obtain a
lock on the entry that is being added to the cache. Since the entry's
normalized DN is used as the lock object, this should be unique, and there
should be no conflicts. It would probably help to add some debug output for
lock failures, so we can see what's going on.

  -- Howard Chu
  Chief Architect, Symas Corp.       Director, Highland Sun
  http://www.symas.com               http://highlandsun.com/hyc
  Symas: Premier OpenSource Development and Support