This fixes#3518, and ensures that we get useful logs and metrics for lots of
things that happen in the background.
(There are certainly more things that happen in the background; these are just
the common ones I've found running a single-process synapse locally).
The only place that was observed was to set the profile. I've made it
so that the profile is set within store.register in the same transaction
that creates the user.
This required some slight changes to the registration code for upgrading
guest users, since it previously relied on the distributor swallowing errors
if the profile already existed.
Move the functions inside the distributor and import them
where needed. This reduces duplication and makes it possible
for flake8 to detect when the functions aren't used in a
given file.
