dpif: New function flow_dump_next_may_destroy_keys().

This new function allows callers to determine whether previously
returned keys will be modified or reallocated on the next call to
dpif_flow_dump_next(). This will be used in a future commit to allow
batched flow deletion by revalidator threads.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>