Remove overhead from unnecessary MDL_context_owner inheritance

Metadata locking uses a class MDL_context_owner:

/**

   An interface to separate the MDL module from the THD, and the rest of the

   server code.

 */

class MDL_context_owner {

...

  virtual int  is_killed() = 0;

  virtual THD* get_thd() = 0;

class THD: public THD_count, /* this must be first */

...

           public MDL_context_owner,

...

  virtual int is_killed() { return killed; }

  virtual THD* get_thd() { return this; }

It appears MDL_context_owner is only used as a base class of a THD object, in effect MDL_context_owner is a THD, as is also seen by the need to implement virtual functions such as get_thd().

Such virtual functions create run-time overhead (is_killed() is a frequently called function, for one), and needlessly complicate the code.

So let's remove this and make it clear in the code that MDL_context_owner is in fact a THD. For example by simply passing in THD instead of MDL_context_owner in the places that need to have a THD, or perhaps upcasting in get_thd(); in any case remove the virtual functions that only have a single implementation across the server code.