Ensure that sends to each uuid occur in order.

Currently, there is no guaranteed ordering which means that e.g. 2
execute_script calls to the same UUID can end up with their code
running out of order.

This adds a per-UUID queue for sending.

Update the README to explain that ordering is preserved but that await
is still needed for error handling.

This change triggers a report of failing tests on Firefox, e.g.

https://community-tc.services.mozilla.com/tasks/csUIoiUEQNCH-T51qoV29Q/runs/0/logs/public/logs/live.log

however I can't find any *new* failures in there, all the ones I looked at on wpt.fyi were already failing.

Fixed: 1350611,1356786
Change-Id: I3055660f52e6a5793ad74a769bb36f32a8e6b501
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3857583


Reviewed-by: Dominic Farolino <dom@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1040244}