As the number of open tables is increased, table lookup
(testing if a table is already open) and in particular
the case when a table is not open, became increasingly more
expensive.

The problem was caused by the open table lookup mechanism,
which was based on traversing a linked list comparing the
file names.

Fixed by storing a pointer to the MYISAM_SHARE on
the designated handler share area, so it could skip
traversing the linked list on open.
And that there are additional changes for have_rtree,
which are not related to the bug.

Now with the patch we can see good performance improvement
in SELECT on huge number of tables for bigger values for
TABLE_OPEN_CACHE and TABLE_DEFINITION_CACHE.