Fixes#8819.
Previously, the process of dropping expired shards according to the
retention policy duration, was managed by two independent goroutines in
the retention policy service. This behaviour was introduced in #2776,
at a time when there were both data and meta nodes in the OSS codebase.
The idea was that only the leader meta node would run the meta data
deletions in the first goroutine, and all other nodes would run the
local deletions in the second goroutine.
InfluxDB no longer operates in that way and so we ended up with two
independent goroutines that were carrying out an action that was really
dependent on each other.
If the second goroutine runs before the first then it may not see the
meta data changes indicating shards should be deleted and it won't
delete any shards locally. Shortly after this the first goroutine will
run and remove the meta data for the shard groups.
This results in a situation where it looks like the shards have gone,
but in fact they remain on disk (and importantly, their series within
the index) until the next time the second goroutine runs. By default
that's 30 minutes.
In the case where the shards to be removed would have removed the last
occurences of some series, then it's possible that if the database was already at its
maximum series limit (or tag limit for that matter), no further new series
can be inserted.
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