revalidator: Refactor ukey creation/lookup.

revalidator: Refactor ukey creation/lookup.

This patch refactors the code around ukey creation and lookup to
simplify the code for callers. A new function ukey_acquire() combines
these functions and attempts to acquire a lock on the ukey. Failure to
acquire a lock on the ukey is usually a sign that another thread is
handling the same flow concurrently, which means the flow does not need
to be handled anyway.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>