Based on #7619 

async's `get_user_id_by_threepid` and its call stack.

…rg-hotfixes

Synapse 1.15.0rc1 (2020-06-09)
==============================

Features
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- Advertise support for Client-Server API r0.6.0 and remove related unstable feature flags. ([\#6585](#6585))
- Add an option to disable autojoining rooms for guest accounts. ([\#6637](#6637))
- For SAML authentication, add the ability to pass email addresses to be added to new users' accounts via SAML attributes. Contributed by Christopher Cooper. ([\#7385](#7385))
- Add admin APIs to allow server admins to manage users' devices. Contributed by @dklimpel. ([\#7481](#7481))
- Add support for generating thumbnails for WebP images. Previously, users would see an empty box instead of preview image. ([\#7586](#7586))
- Support the standardized `m.login.sso` user-interactive authentication flow. ([\#7630](#7630))

Bugfixes
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- Allow new users to be registered via the admin API even if the monthly active user limit has been reached. Contributed by @dkimpel. ([\#7263](#7263))
- Fix email notifications not being enabled for new users when created via the Admin API. ([\#7267](#7267))
- Fix str placeholders in an instance of `PrepareDatabaseException`. Introduced in Synapse v1.8.0. ([\#7575](#7575))
- Fix a bug in automatic user creation during first time login with `m.login.jwt`. Regression in v1.6.0. Contributed by @olof. ([\#7585](#7585))
- Fix a bug causing the cross-signing keys to be ignored when resyncing a device list. ([\#7594](#7594))
- Fix metrics failing when there is a large number of active background processes. ([\#7597](#7597))
- Fix bug where returning rooms for a group would fail if it included a room that the server was not in. ([\#7599](#7599))
- Fix duplicate key violation when persisting read markers. ([\#7607](#7607))
- Prevent an entire iteration of the device list resync loop from failing if one server responds with a malformed result. ([\#7609](#7609))
- Fix exceptions when fetching events from a remote host fails. ([\#7622](#7622))
- Make `synctl restart` start synapse if it wasn't running. ([\#7624](#7624))
- Pass device information through to the login endpoint when using the login fallback. ([\#7629](#7629))
- Advertise the `m.login.token` login flow when OpenID Connect is enabled. ([\#7631](#7631))
- Fix bug in account data replication stream. ([\#7656](#7656))

Improved Documentation
----------------------

- Update the OpenBSD installation instructions. ([\#7587](#7587))
- Advertise Python 3.8 support in `setup.py`. ([\#7602](#7602))
- Add a link to `#synapse:matrix.org` in the troubleshooting section of the README. ([\#7603](#7603))
- Clarifications to the admin api documentation. ([\#7647](#7647))

Internal Changes
----------------

- Convert the identity handler to async/await. ([\#7561](#7561))
- Improve query performance for fetching state from a PostgreSQL database. ([\#7567](#7567))
- Speed up processing of federation stream RDATA rows. ([\#7584](#7584))
- Add comment to systemd example to show postgresql dependency. ([\#7591](#7591))
- Refactor `Ratelimiter` to limit the amount of expensive config value accesses. ([\#7595](#7595))
- Convert groups handlers to async/await. ([\#7600](#7600))
- Clean up exception handling in `SAML2ResponseResource`. ([\#7614](#7614))
- Check that all asynchronous tasks succeed and general cleanup of `MonthlyActiveUsersTestCase` and `TestMauLimit`. ([\#7619](#7619))
- Convert `get_user_id_by_threepid` to async/await. ([\#7620](#7620))
- Switch to upstream `dh-virtualenv` rather than our fork for Debian package builds. ([\#7621](#7621))
- Update CI scripts to check the number in the newsfile fragment. ([\#7623](#7623))
- Check if the localpart of a Matrix ID is reserved for guest users earlier in the registration flow, as well as when responding to requests to `/register/available`. ([\#7625](#7625))
- Minor cleanups to OpenID Connect integration. ([\#7628](#7628))
- Attempt to fix flaky test: `PhoneHomeStatsTestCase.test_performance_100`. ([\#7634](#7634))
- Fix typos of `m.olm.curve25519-aes-sha2` and `m.megolm.v1.aes-sha2` in comments, test files. ([\#7637](#7637))
- Convert user directory, state deltas, and stats handlers to async/await. ([\#7640](#7640))
- Remove some unused constants. ([\#7644](#7644))
- Fix type information on `assert_*_is_admin` methods. ([\#7645](#7645))
- Convert registration handler to async/await. ([\#7649](#7649))

Calls `self.get_success` on all deferred methods instead of abusing `self.pump()`. This has the benefit of working with coroutines, as well as checking that method execution completed successfully.

There are also a few small cleanups that I made in the process.

Based on matrix-org#7619 

async's `get_user_id_by_threepid` and its call stack.