The main benefit of this patch is that it opens the way for
implementing WL7306 which brings significant improvement to
MDL performance/scalability in some scenarios.

The basic idea behind this patch is to change the MDL_map
implementation to use LF_HASH instead of a partitioned HASH container,
where each partition is protected by individual mutexes.

Nice results of such a change:

- Since on systems with atomic support LF_HASH is lock-free,
  MDL_map_partition::m_mutex and potential concurrency bottleneck
  associated with it was removed.
- For the same reason it doesn't make sense to partition LF_HASH.
  So we returned back to the scheme with one hash for the whole
  MDL_map and removed the MDL_map_partition class and the
  mdl_locks_hash_partitions start-up parameter.
- Thanks to the fact that LF_HASH is integrated with LF_ALLOCATOR
  and uses per-thread hazard pointers to avoid objects in the
  hash from being deleted immediately after they were looked up, we
  were able to get rid of all MDL_map/MDL_lock machinery responsible
  for reference counting (i.e. MDL_lock::m_ref_usage/m_ref_release/
  m_version).
- We also no longer need the MDL_map_partition::m_unused_locks_cache
  as LF_ALLOCATOR has its own mechanism for caching objects which are
  expensive to create/destroy.

To support the above changes the following additional steps were taken:

- Since it is tricky to use LF_HASH with objects of different types
  stored in LF_ALLOCATOR, to support these changes we had to get rid
  of MDL_object_lock/MDL_scoped_lock dichotomy. This was done by moving
  out their differences to a MDL_lock_strategy structure which is
  referenced from the MDL_lock object by pointer.
- To make it easier to use LF_HASH with non-trivially copyable objects
  (such as MDL_lock) a new callback "initialize" was added to it. This
  callback allows finishing of initialization of the object provided by
  LF_ALLOCATOR and set element key from the object passed as parameter to
  lf_hash_insert.
  Also LF_HASH was extended to support a user-provided hash function
  such as MurmurHash3 used in the MDL subsystem.
- LF_HASH and LF_ALLOCATOR initialization functions were extended to be
  able to accept callback functions used in them as explicit parameters.
- lf_alloc_direct_free() was fixed to call destructor callback before
  doing my_free() on memory belong to object being freed.

Also the following user visible change was made -- 
--metadata_locks_cache_size and --metadata_locks_hash_instances startup
options and corresponding system variables were declared as deprecated
as they now have no effect.