Optimize open_tables to take O(N) time

See also MDEV-22180 and MDEV-30103 (comment).

find_fk_prelocked_table traverses all opened tables in connection, MDL_context::find_ticket traverses all mdl tickets which is also not less amount of steps. Both functions are called for each table, taking O(tables^2) time total, during opening tables.

Opening a table for an operation like UPDATE or DELETE for a table with 12k child tables takes about 3.5 seconds on my laptop. Making the two mentioned fuinctions noop takes 0.15 seconds.

What should be done

A list traversal should be changed to a hash lookup when a number of tables to open is big.
Hashing is an expensive operation, so for a smaller number of tables (an exact value is to be determined) it should be left as a list traversal.

We can't predict an exact number of tables to open preliminarily: it grows dynamically, once the tables are opened (and triggers/FK relations are checked), so we need a data structure which will also dynamically adapt.