Better support for multitable protocols.

The nest-protocol interaction is changed to better handle multitable
protocols. Multitable protocols now declare that by 'multitable' field,
which tells nest that a protocol handles things related to proto-rtable
interaction (table locking, announce hook adding, reconfiguration of
filters) itself.

Filters and stats are moved to announce hooks, a protocol could have
different filters and stats to different tables.

The patch is based on one from Alexander V. Chernikov, thanks.