ofproto: Fix abstraction of OpenFlow multiple table support.

ofproto: Fix abstraction of OpenFlow multiple table support.

The ofproto code until now has assumed to some extent that the switch has
exactly two tables, one that handles exact-match flow and the other that
handles all other flows.  Commit 840ee2faf "wdp: Add new wdp_class member
function 'get_table_stats'" did part of the work on getting rid of that
assumption.  This commit does the rest of the work.

Now the only assumption left is that a switch has no more than 32 tables.
This seems reasonable enough to me; OpenFlow allows for no more 255.