Support async itemDone on child translator

[AbeJellinek]

@zoe-translates pointed out that you can't set an async itemDone handler on a child translator, like

let translate = Zotero.loadTranslator('web');
translate.setTranslator('...');
translate.setHandler('itemDone', (obj, item) => {
	let abstractNote = (await requestJSON('https://...')).description;
	item.abstractNote = abstractNote;
	item.complete();
}
await translate.translate();

Instead, that code will cause translation to fail (no items returned) when it awaits the HTTP request.

It looks like we already support async handlers (in most cases, at least!). We just weren't awaiting handlers set on child safeTranslators, nor were we incrementing/decrementing async processes in order to wait for the itemDone handler to complete when the child translator doesn't await Item#complete(). This PR fixes those two issues. Client translation tests are passing.

[zoe-translates]

Thank you - this will make writing translator code much easier. Right now quite a lot of "itemDone" handlers make use of the blocking doGet() function that we want to get rid of, but we can't without this fix.