Drupal SQL/rate limiter/retry-failed/command iteration

The work from !23 has been split out into separate MRs for the relevant tickets, in order to keep things atomic: testable/documenable/reviewable etc.

I've decided !23 is too unwieldy. Too difficult to review and merge unrelated changes. The features need to be made more atomic, for review and code forsensic purposes. I've tried to do this work, breaking the code into stacked MRs.

I've taken the code from !23 removed the unrelated CS/Lint changes and unused/untested middlewares, and created PRs comprising what is currently found in !23 before its closure.

A MR each for:

- Drupal SQL transport
- Retry/failed (built on above Drupal SQL MR)
- Rate limiter (built on above Retry/Failed MR)

And extra MRs for

- Retry failed command (built on above Retry/Failed MR)
- The remnants of SQL middlewares. (built on above Drupal SQL MR)

The MRs are Stacked MRs, relying on the parent MR/branch to be merged.

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So far I've added review feedback to the Drupal SQL transport, which needs to be addressed.

I haven't completed a full review of retry/failed yet.

Rate limiter is something that sneaked into !23, without discussion. I've created a new issue and MR containing this code. I don't mind including the rate limiter integration, but it should be done in a proper proposal.