ofp-util: Clean up cookie handling.

ofp-util: Clean up cookie handling.

Commit e72e793 (Add ability to restrict flow mods and flow stats
requests to cookies.) modified cookie handling.  Some of its behavior
was unintuitive and there was at least one bug (described below).
Commit f66b87d (DESIGN: Document uses for flow cookies.) attempted to
document a clean design for cookie handling.  This commit updates the
DESIGN document and brings the implementation in line with it.

In commit e72e793, the code that handled processing OpenFlow flow
modification requests set the cookie mask to exact-match.  This seems
reasonable for adding flows, but is not correct for matching, since
OpenFlow 1.0 doesn't support matching based on the cookie.  This commit
changes to cookie mask to fully wildcarded, which is the correct
behavior for modifications and deletions.  It doesn't cause any problems
for flow additions, since the mask is ignored for that operation.

Bug #9742

Reported-by: Luca Giraudo <lgiraudo@nicira.com>
Reported-by: Paul Ingram <paul@nicira.com>
Signed-off-by: Justin Pettit <jpettit@nicira.com>