* Pull config_dir_path and data_dir_path calculation out of read_config_files

* Pass config_dir_path and data_dir_path into read_config

Add some tests for bad certificates for federation and .well-known connections

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.

Migrate all tests to use the dict-based config format instead of hanging items off HomeserverConfig (#5171)

It's nothing to do with refreshing the certificates. No idea why it was here.

... basically, carry on and fall back to SRV etc.

two reasons for this. One, it saves a bunch of boilerplate. Two, it squashes
unicode to IDNA-in-a-`str` (even on python 3) in a way that it turns out we
rely on to give consistent behaviour between python 2 and 3.

We don't want to be doing .well-known lookups on these guys.

Turns out that the library does a better job of parsing URIs than our
reinvented wheel. Who knew.

There are two things going on here. The first is that, unlike
parse_server_name, URI.fromBytes will strip off square brackets from IPv6
literals, which means that it is valid input to ClientTLSOptionsFactory and
HostnameEndpoint.

The second is that we stay in `bytes` throughout (except for the argument to
ClientTLSOptionsFactory), which avoids the weirdness of (sometimes) ending up
with idna-encoded values being held in `unicode` variables. TBH it probably
would have been ok but it made the tests fragile.

Move the Host header logic down here so that (a) it is used if we reuse the
agent elsewhere, and (b) we can mess about with it with .well-known.

The problem here is that we have cut-and-pasted an impl from Twisted, and then
failed to maintain it. It was fixed in Twisted in
https://github.com/twisted/twisted/pull/1047/files; let's do the same here.

this makes it easier to stub things out for tests.

it is only ever a bytes now, so let's enforce that.

... instead of the matrix_federation_endpoint

* Remove redundant WrappedConnection

The matrix federation client uses an HTTP connection pool, which times out its
idle HTTP connections, so there is no need for any of this business.

* Correctly retry and back off if we get a HTTPerror response

* Refactor request sending to have better excpetions

MatrixFederationHttpClient blindly reraised exceptions to the caller
without differentiating "expected" failures (e.g. connection timeouts
etc) versus more severe problems (e.g. programming errors).

This commit adds a RequestSendFailed exception that is raised when
"expected" failures happen, allowing the TransactionQueue to log them as
warnings while allowing us to log other exceptions as actual exceptions.