* Remove the _all metadata field
This change removes the `_all` metadata field. This field is deprecated in 6
and cannot be activated for indices created in 6 so it can be safely removed in
the next major version (e.g. 7).
Current features (eg. update API) and future features (eg. reindex API)
depend on _source. This change locks down the field so that
it can no longer be disabled. It also removes legacy settings
compress/compress_threshold.
closes#8142closes#10915
Regardless of the outcome of #8142, we should at least enforce that
when _source is enabled, it is sufficient to reindex. This change
removes the excludes and includes settings, since these modify
the source, causing us to lose the ability to reindex some fields.
closes#10814
Jim Ferenczi
Clinton Gormley
Nik Everett
Shane Connelly
Ryan Ernst
Lee Hinman