Initial commit; this doesn't work yet - the LIKE filtering seems too aggressive.
It also needs _do_initial_spam to be aware of prepopulating the whole user_directory_search table with all users...
...and it needs a handle_user_signup() or something to be added so that new signups get incrementally added to the table too.

Committing it here as a WIP

Avoid locking for upsert on pushers tables

Add automagical AS Publicised Group(s)

* replace the upsert into deleted_pushers with an insert
* no need to lock for upsert on pusher_throttle

This PR was against master, not develop :(

This reverts commit 203058a0, reversing
changes made to 552f123b.

Fix auth handler

via registration file "users" namespace:

```YAML
...
namespaces:
  users:
    - exclusive: true
      regex: '.*luke.*'
      group_id: '+all_the_lukes:hsdomain'
...
```

This is part of giving App Services their own groups for matching users. With this, ghost users will be given the appeareance that they are in a group and that they have publicised the fact, but _only_ from the perspective of the `get_publicised_groups_for_user` API.

Start work on avoiding table locks for upserts

Now that _simple_upsert will retry on IntegrityError, we don't need to lock the
table.

wrap the call to _simple_upsert_txn in a loop so that we retry on an
integrityerror: this means we can avoid locking the table provided there is an
unique index.

Bail out early to reduce indentation

Pull out bits of StateStore to a mixin

Declare support for r0.3.0