ofproto-dpif-xlate: Allow translating without side-effects.

ofproto-dpif-xlate: Allow translating without side-effects.

Extend 'may_learn' attribute to also control the treatment of
FIN_TIMEOUT action and asynchronous messages (packet ins,
continuations), so that when 'may_learn' is 'false' and
'resubmit_stats' is 'NULL', no OpenFlow-visible side effects are
generated by the translation.

Correspondingly, add support for one-time asynchronous messages to
xlate cache, so that all side-effects of the translation may be
executed at a later stage.  This will be useful for bundle commits.

Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>