ofproto: Do straightforward removal of asynchronous flow operations.

ofproto: Do straightforward removal of asynchronous flow operations.

Open vSwitch has supported datapaths that cannot update their flow tables
synchronously for many versions.  In that time, I have talked to many
hardware implementers.  None of them has ever mentioned the asynchronous
interface.  Furthermore, the only public hardware implementation of an Open
vSwitch datapath (from Centec), does not use the asynchronous interface.

At the same time, the asynchronous interface makes ofproto hard to read and
hard to understand.  It also makes it hard to maintain and extend.  An
extension in an upcoming commit would be very difficult to implement
asynchronously.

Therefore, this commit begins to remove the asynchronous interface.  This
initial commit does only the most straightforward parts of the removal, the
ones that do not significantly change the structure of the code.  For
example, this commit does not remove the ofoperation or ofopgroup data
structures at the core of the asynchronous interface, but instead reduces
them to a vestigial form: where previously an ofoperation might span
multiple trips through the main loop (if the operation were truly
asynchronous), now it always completes immediately.

The following commit will do more structural changes.  It will also update
all the comments, which are mostly left alone here.

The hope is that this structuring of the asynchronous removal into two
stages will make it easier to understand and review.  If not, the commits
could be squashed.

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