Expose a flush function which proxies to the seq-logger, reprise!

[larenelg]

Moving discussion from a closed PR to here.

Thanks @tjrobinson for pointing out that flush() is no longer accessible, and that is causing issues to developers relying on that function and making them unable to upgrade.

This issue is to document an investigation into what it was used for, and whether or not it should be put back, or if we roll forward.