tag: Retire the venerable tag library.

tag: Retire the venerable tag library.

This patch retires a venerable library whose inception dates before
the first patch of the current repository: tags.  They have served us
well, but their time has come for the reasons listed below.

1) They don't actually help much.
In theory, tags had been used to reduce revalidation necessary when
using bonds, mac-learning, and frequently changing flow tables.  With
bonds and mac-learning, things change happen so rarely that tagging
isn't worth it.  That leaves flow table changes. With the complex flow
tables in my testing, the revalidate_set gets so overwhelmed with
tags, that we end up revalidating every facet every time through the
run loop.  In other words, they tags are giving us no benefit.

2) They complicate the code.
This patch simplifies the code and removes a couple of rather ugly
kludges.

3) They complicated locking once threading hits.
Because of the calculate_flow_tag() function, the table_dpif structure
would require locking in a multi-threaded OVS.  Though this problem
isn't insurmountable, it's annoying and probably would cause lock
contention.

Of course, we could try to work around these problems with a more
advanced tagging infrastructure, but this moves in the opposite of the
direction we should be.  Ideally we'll have a more-or-less stateless
ofproto-dpif supporting a massive number of datapath flows.  Tags (or
facets for that matter) aren't going to work in this new world.

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>