Core: DiskThresholdDecider#remain(...) should take shards relocating away into account

Let the disk threshold decider take into account shards moving away from a node in order to determine if a shard can remain.

By taking this into account we can prevent that we move too many shards away than is necessary.

PR for #8538
can we name this `subtractShardsMovingAway` ? it's clear what it means