Deprecate and remove shared and per index data path

[rjernst]

The shared data path is a feature within Elasticsearch that was previously used by shadow replicas. While shadow replicas were removed in 6.0, the shared data path was kept, for an undocumented capability: to allow per index paths.

The per index data path is a setting which must be relative to the shared data path, and has been used by some (again, undocumented) with multi tenant clusters to provide on disk isolation (in particular, for unique encryption at rest per index). While the desire for such a capability is understandable, Elasticsearch was not designed for multi tenant isolation. This feature doesn't actually provide these users what they desire. There are other indexes and on disk data structures that are outside of the index directory, for example async search results. Additionally, the feature presents similar internal problems as Multiple Data Paths since the system must still track and monitor several disk mounts.

Given the above, we plan to deprecate and remove the shared data path and per index data paths. The feature will be deprecated in 7.x, and blocked from new indices using it in 8.x (though reading indexes from 7.x with the setting will continue to work). Finally the feature will be removed in 9.0. This is a meta issue to track that deprecation and removal.

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)