Cache get_presence storage

http txns: Do not cache error responses

Previously we did. This meant that, amongst other errors, rate-limiting errors
would be cached and prevent messages with that txn ID being sent.

Add db functions needed for room initial sync to slave

Use signedjson.sign instead of syutil.crypto.jsonsign

Functions from syutil.crypto.jsonsign are now available in
signedjson, so use that instead of depending on syutil.

Signed-off-by: Andrew Shadura <andrew@shadura.me>

Fix typo in config comments.

Signed-off-by: Tyler Smith <tylersmith.me@gmail.com>

Update CAPTCHA_SETUP.rst X-Forwarded-For docs

It looks like CAPTCHA_SETUP.rst contains information relevant to an old version of Synapse, but Synapse now has a different approach to configuring use of the X-Forwarded-For header.

Ignore new rejected events when working out forward extremities.

Remove a few aspirational but unused constants

from the Kegan era

Administration functionalities

Bump cache sizes for common membership queries

Measure new device list stuff

Make /keys/changes a bit more performant

Make presence.get_new_events a bit faster