* Pull Sentinel out of LoggingContext

... and drop a few unnecessary references to it

* Factor out LoggingContext.current_context

move `current_context` and `set_context` out to top-level functions.

Mostly this means that I can more easily trace what's actually referring to
LoggingContext, but I think it's generally neater.

* move copy-to-parent into `stop`

this really just makes `start` and `stop` more symetric. It also means that it
behaves correctly if you manually `set_log_context` rather than using the
context manager.

* Replace `LoggingContext.alive` with `finished`

Turn `alive` into `finished` and make it a bit better defined.

... and use it in places where it's trivial to do so.

This will make it easier to pass room versions into the FrozenEvent
constructors.

Lift the restriction that *all* the keys used for signing v2 key responses be
present in verify_keys.

Fixes #6596.

There's an awful lot of deferreds and dictionaries flying around here. The
whole thing can be made much simpler and achieve the same effect.

Some keys are stored in the synapse database with a null valid_until_ms
which caused an exception to be thrown when using that key. We fix this
by treating nulls as zeroes, i.e. they keys will match verification
requests with a minimum_valid_until_ms of zero (i.e. don't validate ts)
but will not match requests with a non-zero minimum_valid_until_ms.

Fixes #5391.

There are a few changes going on here:

* We make checking the signature on a key server response optional: if no
  verify_keys are specified, we trust to TLS to validate the connection.

* We change the default config so that it does not require responses to be
  signed by the old key.

* We replace the old 'perspectives' config with 'trusted_key_servers', which
  is also formatted slightly differently.

* We emit a warning to the logs every time we trust a key server response
  signed by the old key.

Also:
* rename VerifyKeyRequest->VerifyJsonRequest
* calculate key_ids on VerifyJsonRequest construction
* refactor things to pass around VerifyJsonRequests instead of 4-tuples

When handling incoming federation requests, make sure that we have an
up-to-date copy of the signing key.

We do not yet enforce the validity period for event signatures.

The list of server names was redundant, since it was equivalent to the keys on
the server_to_deferred map. This reduces the number of large lists being passed
around, and has the benefit of deduplicating the entries in `wait_on`.

Rather than have three methods which have to have the same interface,
factor out a separate interface which is provided by three implementations.

I find it easier to grok the code this way.

This is a first step to checking that the key is valid at the required moment.

The idea here is that, rather than passing VerifyKey objects in and out of the
storage layer, we instead pass FetchKeyResult objects, which simply wrap the
VerifyKey and add a valid_until_ts field.

Storing server keys hammered the database a bit. This replaces the
implementation which stored a single key, with one which can do many updates at
once.

make sure we store the name of the server the keys came from, rather than the
origin server, after doing a fetch-from-perspectives.

This is a bit fiddly due to the keyring doing weird things with logcontexts.

* Add better diagnostics to flakey keyring test

* fix interpolation fail

* Check logcontexts before and after each test

* update changelog

* update changelog

This is a mixed commit that fixes various small issues

 * print parentheses
 * 01 is invalid syntax (it was octal in py2)
 * [x for i in 1, 2] is invalid syntax
 * six moves

Signed-off-by: Adrian Tschira <nota@notafile.com>

... instead of creating our own special SQLiteMemoryDbPool, whose purpose was a
bit of a mystery.

For some reason this makes one of the tests run slightly slower, so bump the
sleep(). Sorry.

It turns out that the only thing we use the __dict__ of LoggingContext for is
`request`, and given we create lots of LoggingContexts and then copy them every
time we do a db transaction or log line, using the __dict__ seems a bit
redundant. Let's try to optimise things by making the request attribute
explicit.

Fix a bug where we could end up firing off multiple requests for server_keys
for the same server at the same time.

I'm still unclear on what the intended behaviour for
`verify_json_objects_for_server` is, but at least I now understand the
behaviour of most of the things it calls...