Push API should support multiple notifications in a single push (bulk API)

[Half-Shot]

It currently supports one notification but there is some evidence to suggest it would be more efficient to deliver these in bulk transaction-like requests.

[richvdh]

This has interactions with how we manage reliability of push when sygnal is unreachable, or restarted while processing a push request:

Currently synapse makes no attempt to resend pushes which don't get a successful response from sygnal; however that obviously leads to unreliability. Assuming we wanted to fix that, we might need to give pushes transaction ids so that sygnal can de-duplicate. We should consider how we would extend that to transactions which contain multiple pushes. (Although a simple mechanism would be just to give each push within a transaction its own ID.)

[reivilibre]

Each push in a transaction can be identified by (transaction ID, index within the transaction).

I do wonder if this would mean making Sygnal stateful (currently it is virtually stateless).

Also a great chance to fix the issue of not being able to indicate errors with per-push granularity.

[richvdh]

It currently supports one notification

Technically speaking, it supports sending the same notification to multiple devices at a time, via the devices parameter. That said, it's not currently used. It's of vague interest for MSC3013 which would make it totally useless.