The problem was that the internal temporary table created
for information_schema overflow to MyISAM because it has
a row width of > 3000 characters, which filled the in memory
temporary tables.
Fix was to increase size for the heap table.
* Don't write frm for tmp tables
* pass frm image down to open_table_uncached, when possible
* don't use truncate-by-recreate for temp tables - cannot recreate
without frm, and delete_all_rows is faster anyway
