SLOW/CRASHES SEMAPHORE

Problem:

There are 2 lakh tables - fk_000001, fk_000002 ... fk_200000.  All of them
are related to the same parent_table through a foreign key constraint.
When the parent_table is loaded into the dictionary cache, all the child table
will also be loaded.  This is taking lot of time.  Since this operation happens
when the dictionary latch is taken, the scenario leads to "long semaphore wait"
situation and the server gets killed.

Analysis:

A simple performance analysis showed that the slowness is because of the
dict_foreign_find() function.  It does a linear search on two linked list
table->foreign_list and table->referenced_list, looking for a particular
foreign key object based on foreign->id as the key.  This is called two
times for each foreign key object.

Solution:

Change the linked lists table->foreign_list and table_referenced_list to
std::set structures table->foreign_set and table->referenced_set.

rb#5673 approved by Vasil.