Let us access some data members of THD directly, instead of invoking
non-inline accessor functions. Note: my_thread_id will be used instead
of the potentially narrower ulong data type.
Also, let us remove some functions from sql_class.cc that were only
being used by InnoDB or RocksDB, for no reason. RocksDB always had
access to the internals of THD.
Reviewed by: Sergei Golubchik
Tested by: Saahil Alam
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