Make to-device message sending ordered

[ara4n]

This is problematic if you use them for something like matrix-org/matrix-spec-proposals#3401, where it's valid to send a cluster of to-device events at the same time, but causality should be preserved (otherwise you'll receive candidates before invites, etc).

[uhoreg]

https://spec.matrix.org/unstable/client-server-api/#server-behaviour-4 says

... the server should list the pending messages, in order of arrival, in the response body.

So it seems like if you're receiving messages out of order, it's a server issue.

[turt2live]

while the receipt of to-device events is ordered, the sending isn't necessarily guaranteed over federation

[richvdh]

while the receipt of to-device events is ordered, the sending isn't necessarily guaranteed over federation

honestly I kinda thought to-device delivery was ordered over federation in practice (although the spec doesn't say so explicitly).

Significant reordering could cause decryption problems (cf element-hq/element-web#25723), so if not, it's a thing we should fix.