Refactor event storage to prepare for changes in state calculations

Make Client-Server API return 403 for invalid token

Escape label values in prometheus metrics

Fix a class of logcontext leaks

Fix logcontext leak in media repo

Fix a couple of logcontext leaks in unit tests

Make FileResponder.write_to_consumer uphold the logcontext contract

... which were making other, innocent, tests, fail.

Plus remove a spurious unittest.DEBUG which was making the output noisy.

So, it turns out that if you have a first `Deferred` `D1`, you can add a
callback which returns another `Deferred` `D2`, and `D2` must then complete
before any further callbacks on `D1` will execute (and later callbacks on `D1`
get the *result* of `D2` rather than `D2` itself).

So, `D1` might have `called=True` (as in, it has started running its
callbacks), but any new callbacks added to `D1` won't get run until `D2`
completes - so if you `yield D1` in an `inlineCallbacks` function, your `yield`
will 'block'.

In conclusion: some of our assumptions in `logcontext` were invalid. We need to
make sure that we don't optimise out the logcontext juggling when this
situation happens. Fortunately, it is easy to detect by checking `D1.paused`.

Fix incorrect reference to StringIO

This was introduced in 4f2f5171

bump version

* add some doc about wtf this thing does
* pin Twisted to < 18.4
* add explicit dep on six (fixes #3089)

* When creating a new event, cap its depth to 2^63 - 1
* When receiving events, reject any without a sensible depth

As per https://docs.google.com/document/d/1I3fi2S-XnpO45qrpCsowZv8P8dHcNZ4fsBsbOW7KABI

This closes #2602

v1auth was created to account for the differences in status code between
the v1 and v2_alpha revisions of the protocol (401 vs 403 for invalid
tokens). However since those protocols were merged, this makes the r0
version/endpoint internally inconsistent, and violates the
specification for the r0 endpoint.

This might break clients that rely on this inconsistency with the
specification. This is said to affect the legacy angular reference
client. However, I feel that restoring parity with the spec is more
important. Either way, it is critical to inform developers about this
change, in case they rely on the illegal behaviour.

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

fixes #3135

Signed-off-by: Will Hunt <will@half-shot.uk>

add guard for None on purge_history api

remove duplicates from groups tables

Remove redundant call to preserve_fn

Trap exceptions thrown within run_in_background

run config tests on py3

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

Make event properties raise AttributeError instead