ofproto-dpif: Lock rules to prevent eviction.

This patch uses a read-write lock to prevent rules from being evicted
while they're used by child threads.  It also changes the prototypes
of the various rule lookup functions so that the thread safety
analysis can be used to ensure that the locking is handled properly.

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>