Instead of every time a new email pusher is created, as loading jinja2
templates is slow.

Most of the time was spent copying a dict to filter out sentinel values
that indicated that keys did not exist in the dict. The sentinel values
were added to ensure that we cached the non-existence of keys.

By updating DictionaryCache to keep track of which keys were known to
not exist itself we can remove a dictionary copy.

might help us figure out if https://github.com/vector-im/riot-web/issues/3868
has happened.

When a client retries a key upload, don't give an error if the signature has
changed (but the key is the same).

Fixes https://github.com/vector-im/riot-android/issues/1208, hopefully.

As the TCP replication uses a slightly different API and streams than
the HTTP replication.

This breaks HTTP replication.

The cache wrappers had a habit of leaking the logcontext into the reactor while
the lookup function was running, and then not restoring it correctly when the
lookup function had completed. It's all the fault of
`preserve_context_over_{fn,deferred}` which are basically a bit broken.

This prevents unnecessary construction of lists

 * use a valid filter in rest/client/v2_alpha test

Signed-off-by: pik <alexander.maznev@gmail.com>

Signed-off-by: pik <alexander.maznev@gmail.com>

 * add invalid filter tests

Signed-off-by: pik <alexander.maznev@gmail.com>

Due to a failure to instantiate DeferredTimedOutError, time_bound_deferred
would throw a CancelledError when the deferred timed out, which was rather
confusing.

rather than having to instrument everywhere we make a federation call,
make the MatrixFederationHttpClient manage the retry limiter.

The `@cached` decorator on `KeyStore._get_server_verify_key` was missing
its `num_args` parameter, which meant that it was returning the wrong key for
any server which had more than one recorded key.

By way of a fix, change the default for `num_args` to be *all* arguments. To
implement that, factor out a common base class for `CacheDescriptor` and `CacheListDescriptor`.

Fix a bug in ``logcontext.preserve_fn`` which made it leak context into the
reactor, and add a test for it.

Also, get rid of ``logcontext.reset_context_after_deferred``, which tried to do
the same thing but had its own, different, set of bugs.

This was broken when device list updates were implemented, as Mailer
could no longer instantiate an AuthHandler due to a dependency on
federation sending.

This is because it now relies of the caches stream, which only works on
postgres. We are trying to test with sqlite.

Instead of calculating the size of the cache repeatedly, which can take
a long time now that it can use a callback, instead cache the size and
update that on insertion and deletion.

This requires changing the cache descriptors to have two caches, one for
pending deferreds and the other for the actual values. There's no reason
to evict from the pending deferreds as they won't take up any more
memory.