Fix dbcache/entry lock deadlock. If dbcache lock is held, it's

Fix dbcache/entry lock deadlock.  If dbcache lock is held, it's
okay to read and write LDBM specific fields (state, refcnt,
LRU.  The id field, though is read-only once set.
cache_find_entry_dn2id(), hence, does not require any entry locks.
cache_find_entry_id() must do a entry_rdwr_trylock() and back
off if busy.
Add new rdwr lock code with trylock() functionality.
Implement entry_rdwr_trylock().