As mentioned offline:

update_filter is a bit unsatisfactory to me. E.g. it leads to irritation with users (see#1595) and makes typing hard (see#1920)

What this PR does is:

• MakeBaseFilter.__call__ an abstract method and implement two subclasses:MessageFilter andUpdateFilter, each of which will take care of passing the right object tofilter()
• instead of settingupdate_filter, make the filters inherit from those.
• Also move the abstract filter() method fromBaseFilter toMessage/UpdateFilter in order to make typing even more clear. This makes it breaking, but
  • v13 is breaking anyway
  • switching inheritance fromBaseFilter toMessageHandler is not too bad. Leavingupdate_filter = True won't even hurt …

Offline I also mentioned droppingdata_filter, but that was nonsense, as we need that for the short circuit logic of the filters

Closes#1595 and#1596