Port "technical" backends to the new API

[mooskagh]

Recently we've switched from the old API to new API for backends.

The old backends are currently working through old-to-new wrapper.

Eventually, it would be nice to port all backends to the new API, but most importantly, it would be nice to port technical pass-through backends first.

Here are such backends, roughly sorted by the importance of converting to the new API:

1. multiplexing
2. demux
3. check
4. roundrobin
5. recordreplay