ofproto: Collect all rules for an OpenFlow request before acting on any.

ofproto: Collect all rules for an OpenFlow request before acting on any.

An upcoming commit adds support for asynchronous flow table modification.
In an attempt to ensure that the software and hardware flow tables are
properly in sync, that commit limits any given rule to a single outstanding
operation at a time.  It does so by figuring out all of the rules that an
OpenFlow request will affect before modifying any of them, and then
deferring the request if it will affect any rules that have ongoing
operations.  This commit is a step in that direction.