Continuous throttling policy for wdtk-wikibaseapi

[wetneb]

Background: https://phabricator.wikimedia.org/T240442

Bots currently edit at full speed until the lag reaches 5 seconds. They should instead slow down as the lag rises. We need to introduce a continuous throttling mechanism in WTDK.

[wetneb]

Stalled by https://phabricator.wikimedia.org/T240442#5851541

[wetneb]

Looking at the phabricator task, we could simply expect that throttling is going to be handled server-side, so we should just make sure we handle 429 status codes and retry-after headers appropriately (suggested by @tfmorris in https://github.com/OpenRefine/OpenRefine/issues/2304).