Object Store HTTP Rate Limiting

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
I'd like to limit the rate of requests I make to S3, to avoid running into throttling on their side.

**Describe the solution you'd like**
There could be a wrapper, similar to `ThrottledStore`, that puts requests in a queue and executes them up to specified throughput.

**Describe alternatives you've considered**
Rate limiting could be baked into ObjectStore implementations that need it, with rules and configuration that apply to them only. This might be better if there is a wide variety of rate limiting rules between different vendors. On the other hand, this is worse if, say, S3-compatible vendors have different rules between them.

The issue names no files or tests; start by reading the existing ThrottledStore implementation and the ObjectStore request APIs. Determine whether a configurable queue can provide the requested throughput for S3 requests, and define tests that demonstrate the rate limit and completion behavior.

Written by the indexing model from the issue text.