Apply one AIP-158 keyset pagination convention (page_size/page_token/
next_page_token, opaque cursor, not offset) to every list/search RPC:
ListMessages, ListFolders, ListRules, ListEvents, ListContacts,
SearchMessages, and reconcile Audit.ListRecords off its unstable
limit/offset. A malformed page_token returns a typed VALIDATION_FAILED
ErrorInfo (InvalidArgument). Regenerate the nuncio.v1 golden descriptor.
Refs #275