lowercasing table name".

In lower_case_table_names > 0 mode some queries to I_S left entries
with incorrect key in table definition cache. This wasted memory and
caused some of the further queries to I_S to produce stale results
in cases when table definition was changed by a DDL statement.
Also in combination with similar problem in CREATE TABLE (which also
has peeked into table definition cache using non-normalized key) this
issue led to to spurious ER_TABLE_EXISTS_ERROR errors when one tried
to create a table with the same name as a previously existing but
dropped table (assuming that table name contained characters in upper
case).

This problem occured due to fact that fill_schema_table_from_frm()
was not properly normalizing (lowercasing) database and table names
which it used for lookups in table definition cache.

This fix adds proper normalization to this function. It also solves
similar problem in CREATE TABLE's code by ensuring that it uses
properly normalized version of table name when it peeks into table
definition cache instead of non-normalized one.