MDEV-6089 - MySQL WL#7305 "Improve MDL scalability by using lock-free hash"
MDEV-6089 - MySQL WL#7305 "Improve MDL scalability by using lock-free hash"

Simplified MDL_object_lock and MDL_scoped_lock dichotomy so that they can be
stored in LF_HASH. This was done by moving out their differences to a
MDL_lock_strategy structure which is referenced from the MDL_lock object by
pointer.

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