Provide a way to make message order explicit when sending (rather than assuming that /send updates the DAG before returning)

[ara4n]

In other words, if you do a /send which returns a 200 OK and immediately do another /send, does the spec guarantee ordering such that the second follows the first in the DAG? Or is there any risk that they will end up with the same parent node in the DAG? If the latter, how are we ever meant to get consistent ordering for a given client?

If the spec guarantees that the HS's copy of the room DAG is updated before it returns 200 OK, we should presumably spell it out in the spec.

[ara4n]

Having briefly conferred with @erikjohnston, it sounds like the design is deliberately to not force HS nodes to linearise the DAG every time they receive a message (letting things fan out across multiple nodes instead).

So, it seems like instead we're missing an API to let /send state which event a given event is intended to follow. The HS could then use this to guide how it wires the DAG.

(I'm a bit scared that it's taken us 3 years to notice this is missing, but hey...)