[RFC] classifier: Add support for conjunctive matches.

A "conjunctive match" allows higher-level matches in the flow table, such
as set membership matches, without causing a cross-product explosion for
multidimensional matches.  Please refer to the documentation that this
commit adds to ovs-ofctl(8) for a better explanation, including an example.

Issues:

- Until now the conceptual model of a cls_rule has been that it is
  immutable while it is in a classifier.  This commit adds a "conjunctive
  match" (optional) to each cls_rule, and makes the new member safely
  mutable while it is in a classifier.  This might be a conceptual failing
  bad enough to need fixing; I am not sure.

- Needs some real tests; you can run the "test-conjunction" script inside
  "make sandbox", for now.

- The code needs some more comments.
---
v1->v2:
  - Use 1-based dimension numbers in formatted syntax, e.g. 1/2 and 2/2,
    not 0/2 and 1/2.
  - Add new conj_id field instead of overwriting reg0.
  - Since priority is now an 'int', get rid of awkward +1s and comparisons
    on priorities in classifier_lookup().
  - Fix memory leak in classifier_replace().
  - Modify conj_id in-place in classifier_lookup() instead of copying
    entire flow.
  - Remove prototype for nonexistent cls_rule_init_conjunction().
  - Fix memory leak in classifier_lookup(), and eliminate memory allocation
    in the common case of few conjunctive matches.

v2->v2.1:
  - Rebase.

v2.1->v3:
  - Fix race condition modifying rules to add or remove conjunctions.