lf_hash changes, in lfind()

casts, etc

real changes are:
* remove one retry, it is enough to check for DELETED
  after the key is read
* advance 'head' pointer when we see a dummy node to have
  shorter retries
lf_hash_iterate() function
MDEV-6089 - MySQL WL#7305 "Improve MDL scalability by using lock-free hash"

This is prerequisite for MDEV-6089. Added lf_hash_search_using_hash_value(),
which can reuse previously calculated hash value.