dpif-netdev: Polling threads directly call ofproto upcall functions.

Typically, kernel datapath threads send upcalls to userspace where
handler threads process the upcalls. For TAP and DPDK devices, the
datapath threads operate in userspace, so there is no need for
separate handler threads.

This patch allows userspace datapath threads to directly call the
ofproto upcall functions, eliminating the need for handler threads
for datapaths of type 'netdev'.

Signed-off-by: Ryan Wilson <wryan@nicira.com>
Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>