ofproto-dpif: Run fast internally.

ofproto-dpif: Run fast internally.

ofproto-dpif is responsible for quite a few book keeping tasks in
addition to handling flow misses.  Many of these tasks (flow
expiration, flow revalidation, etc) can take many hundreds of
milliseconds, during which no misses can be handled.  The ideal
long term solution to this problem, is to isolate flow miss
handling into it's own thread.  However, for now this patch
provides a 5% increase in TCP_CRR performance, and smooths out
results during revalidations.

Signed-off-by: Ethan Jackson <ethan@nicira.com>