feat(stovepipe): coalesce backlog in process controller (#324)

## What

This PR begins implementing core logic of the Process controller to
supersede older requests as new ones come in.


https://github.com/uber/submitqueue/blob/main/doc/rfc/stovepipe/steps/process.md#process-algorithm

- `accepted` requests get compared against the queue's latest request ID
value
- If a request is outdated compared to the queue's last ID, mark it
superceded (this PR)
- Otherwise, we'll build it now or re-check it later when a build slot
is free (logic to come)
- `superseded` / `processing` → ack no-op
- Retryable load for request/queue not-found

## Why
Implementing
https://github.com/uber/submitqueue/blob/main/doc/rfc/stovepipe/steps/process.md
step. Building out logic to accept incoming requests and narrow down
builds to only the latest head.

## Test plan

- [x] `//stovepipe/controller/process:go_default_test`
- [x] `//stovepipe/controller:go_default_test`
- [x] `//test/integration/stovepipe:go_default_test`

Author: mnoah1