Auto unlock indices when disk space is freed

[cdalexndr]

Describe the feature:
When running out of space indices go to read-only, causing errors:
cluster_block_exception: index [metricbeat-7.3.0-2019.09.19-000001] blocked by: [FORBIDDEN/12/index read-only / allow delete (api)];
After resolving the space issue, the indices are still locked and require manual intervention: https://discuss.elastic.co/t/forbidden-12-index-read-only-allow-delete-api/110282/4

If the indices are auto-locked when disk is full, then they should be auto-unlocked when disk space is available again, so no manual intervention will be required to restore the system to a working state.

[ywelsch]

Already implemented and will be released in ES 7.4, see #42559 and #39334