Allow error strings from spam checker

Fix stackoverflow and logcontexts from linearizer

log pdu_failures from incoming transactions

A logformatter which includes the stack when the exception was caught when
logging exceptions.

make sure we have the relevant fields before we try to log them.

Fix group stream replication

The stream update functions expect the storage function to return a list
of tuples.

fix a logcontext leak in read receipt handling

1. make it not blow out the stack when there are more than 50 things waiting
   for a lock. Fixes https://github.com/matrix-org/synapse/issues/2505.

2. Make it not mess up the log contexts.

... even if we have no EDUs.

This appears to have been introduced in
47689929.

Initial Group Implementation

Support for channel notifications

Unfreeze event before serializing with ujson

Rather than making the condition directly require a specific power
level. This way the level require to notify a room can be configured
per room.

Ignore incoming events for rooms that we have left

also update copyright

fed server: process PDUs for different rooms in parallel

Fed server: use a linearizer for ongoing transactions

With luck, this will give a real-time improvement when there are many rooms and
the server ends up calling out to fetch missing events.

We don't want to process the same transaction multiple times concurrently, so
use a linearizer.

fed server: refactor on_incoming_transaction

Move as much as possible to after the have_responded check, and reduce the
number of times we iterate over the pdu list.

Fed server: Move origin-check code to _handle_received_pdu

The response-building code expects there to be an entry in the `results` list
for each entry in the pdu_list, so the early `continue` was messing this
up. That doesn't really matter, because all that the federation client does is
log any errors, but it's pretty poor form.