ofproto-dpif-upcall: Remove the flow_dumper thread.

Previously, we had a separate flow_dumper thread that fetched flows from
the datapath to distribute to revalidator threads. This patch takes the
logic for dumping and pushes it into the revalidator threads, resulting
in simpler code with similar performance to the current code.

One thread, the "leader", is responsible for beginning and ending each
flow dump, maintaining the flow_limit, and checking whether the
revalidator threads need to exit. All revalidator threads dump,
revalidate, delete datapath flows and garbage collect ukeys.

Co-authored-by: Joe Stringer <joestringer@nicira.com>
Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>