ovs-thread: Implement OVS specific barrier.

Non-leader revalidator thread uses pthread_barrier_* functions in their
main loop to synchronize with leader thread.  However, since those threads
only call poll_block() intermittently, the poll interval check in
poll_block() can wrongly take the time since last call as poll interval
and issue the following warnings:

"Unreasonably long XXXXms poll interval".

To prevent it, this commit implements the barrier struct and operations
for OVS which allow thread to block on barrier via poll_block().

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>