WL#2903: Create list of tables used in transaction

The task is to create a list of tables used in a transaction.
This is needed to be able to check in cases like 'drop table' 
if there exist a THD that indirectly uses a table and allow a 
thread to open a name-locked table if it's already in use by 
the same thread.

Rationale
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Code cleanup task requested by Monty.