* master: cap writable volumes per collection via -maxWritableVolumes
Some deployments serialize disk writes (e.g. an Unraid array where
concurrent writes to multiple disks collapse throughput). With more
than one writable volume in a collection, the master's weighted pick
spreads assignments over all of them, so several disks get written at
once.
-maxWritableVolumes=N narrows the pick candidates to the N fullest
writable volumes, so a collection's writes concentrate on one (or few)
volumes at a time. 0 keeps the current spread-everywhere behavior.
Generated with [Devin](https://devin.ai)
Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>
* master: give master.follower a maxWritableVolumes default
* master: cap writable volumes inside the requested location
* master: pick the capped volumes without sorting the writable list
* master: assert the capped candidate set instead of random pick outcomes
* master: describe -maxWritableVolumes as a per-layout cap
---------
Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>
