ofproto: Encapsulate classifier tables in new "struct oftable".

ofproto: Encapsulate classifier tables in new "struct oftable".

So far, each OpenFlow table has just been a classifier.  Upcoming
commits will add more data for each OpenFlow table, so this commit
encapsulates the struct classifier in a higher-level structure
to provide a good place to keep that data.

Some of the new functions introduced in this commit are trivial, but they
will become more elaborate in upcoming commits.

Signed-off-by: Ben Pfaff <blp@nicira.com>