Scrolling was only implemented for the `SqlAction` (not jdbc or cli)
and it was implemented by keeping request state on the server. On
principle we try to avoid adding extra state to elasticsearch where
possible because it creates extra points of failure and tends to
have lots of hidden complexity.
This replaces the state on the server with serializing state to the
client. This looks to the user like a "next_page" key with fairly
opaque content. It actually consists of an identifier for the *kind*
of scroll, the scroll id, and a base64 string containing the field
extractors.
Right now this only implements scrolling for `SqlAction`. The plan
is to reuse the same implementation for jdbc and cli in a followup.
This also doesn't implement all of the required serialization.
Specifically it doesn't implement serialization of
`ProcessingHitExtractor` because I haven't implemented serialization
for *any* `ColumnProcessors`.
Original commit: elastic/x-pack-elasticsearch@a8567bc5ec
SQL relies on being able to fetch information about fields from
the cluster state and it'd be disasterous if that information
wasn't available. This should catch that.
Original commit: elastic/x-pack-elasticsearch@1a62747332
Nik Everett