This change adds the unfollow action for CCR follower indices.
This is needed for the shrink action in case an index is a follower index.
This will give the follower index the opportunity to fully catch up with
the leader index, pause index following and unfollow the leader index.
After this the shrink action can safely perform the ilm shrink.
The unfollow action needs to be added to the hot phase and acts as
barrier for going to the next phase (warm or delete phases), so that
follower indices are being unfollowed properly before indices are expected
to go in read-only mode. This allows the force merge action to execute
its steps safely.
The unfollow action has three steps:
* `wait-for-indexing-complete` step: waits for the index in question
to get the `index.lifecycle.indexing_complete` setting be set to `true`
* `wait-for-follow-shard-tasks` step: waits for all the shard follow tasks
for the index being handled to report that the leader shard global checkpoint
is equal to the follower shard global checkpoint.
* `pause-follower-index` step: Pauses index following, necessary to unfollow
* `close-follower-index` step: Closes the index, necessary to unfollow
* `unfollow-follower-index` step: Actually unfollows the index using
the CCR Unfollow API
* `open-follower-index` step: Reopens the index now that it is a normal index
* `wait-for-yellow` step: Waits for primary shards to be allocated after
reopening the index to ensure the index is ready for the next step
In the case of the last two steps, if the index in being handled is
a regular index then the steps acts as a no-op.
Relates to #34648
Co-authored-by: Martijn van Groningen <martijn.v.groningen@gmail.com>
Co-authored-by: Gordon Brown <gordon.brown@elastic.co>
Martijn van Groningen