This reverts commit ec716a35.

The field is never read from, and all the opportunities given to populate it are not utilized. It should be very safe to remove this.

Because it was complicated and annoyed me. I suspect this will be more
efficient too.

on_notifier_poke no longer runs synchonously, so we have to do a different hack
to make sure that the replication data has been sent. Let's actually listen for
its arrival.

Linearizer was effectively a Limiter with max_count=1, so rather than
maintaining two sets of code, let's combine them.

Fixes #3570

This is more involved than it might otherwise be, because the current
implementation just drops its logcontexts and runs everything in the sentinel
context.

It turns out that we aren't actually using a bunch of the functionality here
(notably suppress_failures and the fact that Distributor.fire returns a
deferred), so the easiest way to fix this is actually by simplifying a bunch of
code.