allow applying retention at different interval than compaction with a config

[sandeepsukhani]

What this PR does / why we need it:
When retention is enabled, we apply it with each compaction run. This causes a lot of resources to be wasted in a big Loki cluster which also causes compaction to not be able to keep up. Since applying retention is not as critical as compacting the data for better query performance, the apply retention interval can be set to a higher value than the compaction interval.

Special notes for your reviewer:
To keep it backwards compatible, I have allowed setting the apply retention interval to 0, which is the default. A zero value here means applying retention at the same time as compaction.
When a non-zero value is set, it should be a multiple of compaction interval because we would always apply retention when running compaction which is optimum and a pre-requisite for applying retention.

Checklist

• Add an entry in the CHANGELOG.md about the changes.

[cyriltovena]

LGTM

Not sure it needs to be a multiple, I guess this is a best effort.

[sandeepsukhani]

Thanks for the review!

LGTM

Not sure it needs to be a multiple, I guess this is a best effort.

yeah it is a best-effort but requiring it to be multiple aligns the expectations of the user with the internal working of the compactor.