Zap verification API

The metadata data structure format is getting updated, which opens up a few new apis that I am considering: ``` uint64_t ndb_note_meta_get_flags() uint64_t ndb_note_meta_set_flags(uint64_t flag) ``` where one of the flags could be: `NDB_NOTE_FLAG_ZAP_VERIFIED` once the write thread notices this flag is toggled to true on the zap note metadata, then we can update the zap counter on the zap target metadata. This would provide a super robust way of counting zaps