ofproto: Add extra comments and checking for expiring a pending rule.
A given rule may only have one pending operation at a time, so when an
operation is pending we must not allow a flow expiration to be started on
that rule.
This doesn't fix a user-visible bug in ofproto-dpif because ofproto-dpif
always completes operations immediately, that is, no operations will be
pending when expiration runs. (Technically there is a bug if the user
runs "ovs-appctl ofproto/clog", but that feature is for debugging only and
there is no reason for a user to ever run it.)
Signed-off-by: Ben Pfaff <blp@nicira.com>