lib/classifier: Rename 'cls_subtable_cache' as 'cls_subtables'.

'cache' gives an inexact connotation, as the list is always expected
to be in order and contain pointers to all the subtables.

The struct cls_subtables fields are are also renamed to be more readable.

struct cls_classifier fields 'subtables' is remamed to 'subtables_map' and
'subtables_priority' is renamed to 'subtables',

There are no functional changes in this patch.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>