lib/classifier: Simplify subtable array.

Do not cache the 'tag' and 'max_priority' in the subtable array.  This
makes later changes to classifier easier.

Change CLS_SUBTABLES_FOR_EACH to iterate with a regular array index.

Also makes the 'cls_subtables*' functions to always leave the
subtables array in a consistent state.  This includes the new
cls_subtables_insert() function and removal of the old
cls_subtables_push_back() function.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Co-authored-by: Ethan Jackson <ethan@nicira.com>
Signed-off-by: Ethan Jackson <ethan@nicira.com>