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.
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