upcall-latency-benchmark: fix syntax error
OVSDriver: automatically revalidate after a timeout

This makes IVS work again with the reference implementation and Floodlight.
These controllers don't send barriers, and IVS previously only revalidated the
kernel flow table after a barrier.
Merge into master from pull request #224:
OVSDriver: automatically revalidate after a timeout (https://github.com/floodlight/ivs/pull/224)