WL#2903: Create list of tables used in transaction

Affects: Server-7.1   —   Status: Un-Assigned   —   Priority: Medium

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.

Code cleanup task requested by Monty.