Protect storage servers from running out of memory

A sufficiently large read rate to storage servers could cause read queues to grow and exhaust a storage servers memory. We should implement protections from this.

This is a similar issue to #2792, which describes an analogous need for the proxy.

Start by reading this issue alongside the analogous proxy request in #2792. Trace how storage-server read queues grow under a high read rate and identify the existing memory-management and protection paths. Done should mean sustained large read rates cannot exhaust storage-server memory, with tests demonstrating the protection.

Written by the indexing model from the issue text.