I got rid of the `SYNAPSE_USE_FROZEN_DICTS` environment variable because
it will be overridden by the Synapse worker apps anyway and if we want
to support `SYNAPSE_USE_FROZEN_DICTS`, it should be in
`synapse/config/server.py`. It's also not documented so I'm assuming no
one is using it anyway.

Spawning from looking at the frozen dict stuff during the review of
https://github.com/element-hq/synapse/pull/18103#discussion_r1935876168

When a user sends a state event, do not cancel their own delayed events
for the same piece of state.

For context, see [the relevant section in the
MSC](https://github.com/matrix-org/matrix-spec-proposals/blob/a09a883d9a013ac4b6ffddebd7ea87a827d211b9/proposals/4140-delayed-events-futures.md#delayed-state-events-are-cancelled-by-a-more-recent-state-event).

It's mostly used to track accepting invites, not actually blocking joins,
so it should be able to track everyone.

This reverts commit 5497dc36.

This reverts commit 8681621d.

It's mostly used to track accepting invites, not actually blocking joins,
so it should be able to track everyone.

We do a few things in this PR to better support caching:

1. Change `Cache-Control` header to allow intermediary proxies to cache
media *only* if they revalidate on every request. This means that the
intermediary cache will still send the request to Synapse but with a
`If-None-Match` header, at which point Synapse can check auth and
respond with a 304 and empty content.
2. Add `ETag` response header to all media responses. We hardcode this
to `1` since all media is immutable (beyond being deleted).
3. Check for `If-None-Match` header (after checking for auth), and if it
matches then respond with a 304 and empty body.

---------

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

If one workflow is successful and one fails, currently that is reported
as success.

---------

Signed-off-by: Olivier 'reivilibre <oliverw@matrix.org>

Closes: https://github.com/element-hq/synapse/issues/17926

Bumps [anyhow](https://github.com/dtolnay/anyhow) from 1.0.96 to 1.0.97.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a
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<blockquote>
<h2>1.0.97</h2>
<ul>
<li>Documentation improvements</li>
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<li><a
href="https://github.com/dtolnay/anyhow/commit/bfb89ef244fa60af17fb844dc3bddf4b62e4ac9f"><code>bfb89ef</code></a>
Release 1.0.97</li>
<li><a
href="https://github.com/dtolnay/anyhow/commit/c7fca9b08627fe0c4034a3fd38595ffd7dcc8e0e"><code>c7fca9b</code></a>
Ignore elidable_lifetime_names pedantic clippy lint</li>
<li><a
href="https://github.com/dtolnay/anyhow/commit/427c0bb0f34d5da378112f6a19db9efebfd0e40a"><code>427c0bb</code></a>
Point standard library links to stable</li>
<li>See full diff in <a
href="https://github.com/dtolnay/anyhow/compare/1.0.96...1.0.97">compare
view</a></li>
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