tests: Add command to purge revalidators of flows.

This patch adds a new 'ovs-appctl revalidator/purge' command which
flushes all flows from all datapaths, and updates the revalidator
udpif_key cache at the same time.

Update the ofproto-dpif fragment tests which may fail when ukeys are
created from handler threads.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>