…Step (elastic#128361)

The backing indices of a time series data streams (TSDS) have time ranges (start_time & end_time) and they include documents that belong to these time ranges.

To ensure that we will not unfollow a leader TSDS index before the indexing is complete, we need to add a WaitUntilTimeSeriesEndTimePassesStep to the unfollow action. This will ensure that we will only unfollow after the end_time has passed.

This creates some weird semantics with the combination of the rollover and the unfollow. Because we need the rollover of the leader index to finalise the end_time but the unfollow action is injected before the rollover. However, this should be fine, because the leader index will skip the unfollow action so it will rollover and finalise the end_time and the follower index will wait the end_time to pass before it unfollows. Rolling over the follower index will have no effect since it’s already rolled over.

(cherry picked from commit ed7f2ca)

# Conflicts:
#	x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/UnfollowAction.java