mediawiki.api: Prevent misusing #saveOptions

mediawiki.api: Prevent misusing #saveOptions

Task T214963 is about how we misused #saveOptions in VisualEditor and
made MediaWiki sad. I'm not sure whether we should fix the issues
there or here, but it seems like the mistakes would be easy to make in
other software, so let's try here first and see what folks think about
it.

* Do not send action=options API requests for IP users
* Wait for the previous request to finish before sending another

Bug: T214963
Change-Id: I85cfc6b5829bcd96e6245431cd979c24630a8fd8