First some background: StreamChangeCache is used to keep track of what "entities" have 
changed since a given stream ID. So for example, we might use it to keep track of when the last
to-device message for a given user was received [1], and hence whether we need to pull any to-device messages from the database on a sync [2].

Now, it turns out that StreamChangeCache didn't support more than one thing being changed at
a given stream_id (this was part of the problem with #7206). However, it's entirely valid to send
to-device messages to more than one user at a time.

As it turns out, this did in fact work, because *some* methods of StreamChangeCache coped
ok with having multiple things changing on the same stream ID, and it seems we never actually
use the methods which don't work on the stream change caches where we allow multiple
changes at the same stream ID. But that feels horribly fragile, hence: let's update
StreamChangeCache to properly support this, and add some typing and some more tests while
we're at it.

[1]: https://github.com/matrix-org/synapse/blob/release-v1.12.3/synapse/storage/data_stores/main/deviceinbox.py#L301
[2]: https://github.com/matrix-org/synapse/blob/release-v1.12.3/synapse/storage/data_stores/main/deviceinbox.py#L47-L51

I cracked, and added some type definitions in synapse.storage.

Ensure good comprehension hygiene using flake8-comprehensions.

Add typing information to the spam checker modules.

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

This looks like it got half-killed back in #888.

Fixes #6567.

... and update INSTALL.md to include py3.8.

We'll also have to update the buildkite pipeline to run it

* update version of black and also fix the mypy config being overridden

Add a linting script that enforces all boolean values in the default config be lowercase.

This has annoyed me for a while so I decided to fix it.

* Fix presence timeouts when synchrotron restarts.

Handling timeouts would fail if there was an external process that had
timed out, e.g. a synchrotron restarting. This was due to a couple of
variable name typoes.

Fixes #3715.

* remove 2.7 from CI and publishing

* fill out classifiers and also make it not be installed on 3.5

* some minor bumps so that the old deps work on python 3.5

* Pin eliot to <1.8 on python 3.5.2

Fixes https://github.com/matrix-org/synapse/issues/5199

* Add support for 'markers' to python_dependencies

* tell xargs not to strip quotes

pip 19.1 otherwise complains about "editable mode is not supported for
pyproject.toml-style projects"