Add experimental `org.matrix.unstable.to_device_limit` to sync filter

[MatMaul]

Currently the name of the filter parameter is org.matrix.unstable.to_device_limit since it is not in the spec, and a MSC has not been defined yet.

The goal is to do some testing with clients around the usefulness of it, and write a proper MSC if it ends up useful.

[erikjohnston]

Couple of questions here, probably more aimed at the wider @matrix-org/synapse-core team:

1. Is this the right name for the /sync param? Do we want to preface it with org.matrix.unstable.*?

2. I'm not sure we want to enable this on matrix.org (except maybe temporarily for a day), in case clients accidentally start using it for real (yes, even with the name...). I wonder if we should also add an option to only enable it for certain users? But that is only worthwhile if we want to test it on matrix.org accounts

[clokep]

Is this the right name for the /sync param? Do we want to preface it with org.matrix.unstable.*?

It should be namespaced IMO. If we don't have an MSC yet then something like org.matrix.unstable.to_device_limit seems reasonable to me.

I'm not sure we want to enable this on matrix.org (except maybe temporarily for a day), in case clients accidentally start using it for real (yes, even with the name...).

This has generally been how we (try to) treat unstable features.

[MatMaul]

According to the discussion I changed to org.matrix.unstable prefix instead of do_not_use.

[MatMaul]

We are waiting for the person that want to experiment with that to come back from holidays to take a deploy decision.

[richvdh]

I'm going to close this for now as it's bitrotting and not making any progress. We can reopen once we want to resume work on it.