Improvements to initial /syncs

This helps to ensure we don't completely starve other requests.

Don't bother copying records on parent context

	Atomically persit push actions when we persist the event

Update documentation

Derive macaroon_secret_key from signing key.

Don't load all ephemeral state for a room on every sync

Remove pyc files before running tests

This reverts commit 1d19a5ec.

iOS Console is apparently relying on these paths.

Unfortunately, there are people that are running synapse without a
`macaroon_sercret_key` set. Mandating they set one is a good solution,
except that breaking auto upgrades is annoying.

Fix up logcontexts

Add metrics to pushers

Reject additional path segments