Allow more control of index.merge.policy.deletes_pct_allowed

[n0othing]

Today, the default merge policy allows you to control the maximum percentage of deleted documents that is tolerated in an index via index.merge.policy.deletes_pct_allowed. This value defaults to 33% and allows users to configure anywhere between 20% - 50%.

The 20% floor is in place to avoid overwhelming nodes with excessive CPU usage and disk I/O. However, at scale, this 20% could translate to TBs of wasted space. Some users may have really performant hardware and be ok with trading extra CPU/disk I/O in exchange for more free disk space.

[elasticmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[AlexanderGunnarssonMW]

We are also interested in this. We are currently scaling up our cluster based on disk usage and have spare CPU and would benefit from a value lower than 20%.