c.f. #8021 

A lot of the code here is to change the `Completed 200 OK` logging to include the request URI so that we can drop the `Sending request...` log line.

Some notes:

1. We won't log retries, which may be confusing considering the time taken log line includes retries and sleeps.
2. The `_send_request_with_optional_trailing_slash` will always be logged *without* the forward slash, even if it succeeded only with the forward slash.

* Change default log config to buffer by default.

This batches up writes to the filesystem, which is more efficient for
disk I/O. This means that it can take some time for logs to get written
to disk. Note that ERROR logs (and above) immediately flush the buffer.

This only effects new installs, as we only write the log config if
started with `--generate-config` (in the same way we do for generating
signing keys).

* Default to keeping last 4 days of logs.

This hopefully reduces the amount of logs kept for new servers. Keeping
the last 1GB of logs is likely overkill for new servers, but equally may
not be enough for busy ones.

Instead, we keep the last four days worth of logs, enough so that admins
can investigate any problems that happened over e.g. a long weekend.

Hopefully this mostly speaks for itself. I also did a bit of cleaning up of the
error handling.

Fixes #8047

I think this would have caught all the cases in
https://github.com/matrix-org/synapse/issues/7642 - and I think a 500 makes
more sense here than a 403

With an undocumented configuration setting to enable them for specific users.

This solves the problem that the first few lines are logged twice on matrix.org. Hopefully the comments explain it.

This PR:

* Reduces the amount of noise in the `check-newsfragment` CI output by hiding the dependency installation output by default.
* Prints a link to the changelog/debian changelog section of the contributing guide if an error is found.

This has long been something I've wanted to do. Basically the `Daemonize` code
is both too flexible and not flexible enough, in that it offers a bunch of
features that we don't use (changing UID, closing FDs in the child, logging to
syslog) and doesn't offer a bunch that we could do with (redirecting stdout/err
to a file instead of /dev/null; having the parent not exit until the child is
running).

As a first step, I've lifted the Daemonize code and removed the bits we don't
use. This should be a non-functional change. Fixing everything else will come
later.

We've [decided](https://github.com/matrix-org/synapse/issues/5253#issuecomment-665976308) to remove the signature check for v1 lookups.

The signature check has been removed in v2 lookups. v1 lookups are currently deprecated. As mentioned in the above linked issue, this verification was causing deployments for the vector.im and matrix.org IS deployments, and this change is the simplest solution, without being unjustified.

Implementations are encouraged to use the v2 lookup API as it has [increased privacy benefits](https://github.com/matrix-org/matrix-doc/pull/2134).

`StatsHandler` handles updates to the `current_state_delta_stream`, and updates room stats such as the amount of state events, joined users, etc.

However, it counts every new join membership as a new user entering a room (and that user being in another room), whereas it's possible for a user's membership status to go from join -> join, for instance when they change their per-room profile information.

This PR adds a check for join->join membership transitions, and bails out early, as none of the further checks are necessary at that point.

Due to this bug, membership stats in many rooms have ended up being wildly larger than their true values. I am not sure if we also want to include a migration step which recalculates these statistics (possibly using the `_populate_stats_process_rooms` bg update).

Bug introduced in the initial implementation https://github.com/matrix-org/synapse/pull/4338.