ofproto-dpif-upcall: Remove the dispatcher thread.

With the foundation laid in previous commits, this commit
removes the 'dispatcher' thread by allowing 'handler'
threads to read upcalls directly from dpif.

This commit significantly simplifies the flow miss handling
code and brings slight improvement to flow setup rate.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>