Introduce an actual Rerun Archive format (.rra), banish multi-recordings RRD

[teh-cmc]

Today, an RRD stream (which might be coming from a file (or 10) or a socket or whatever else) is allowed to contain any number and any type (i.e. data vs. blueprint) of recordings within it.

This means that everything that handles RRD streams has to be able to deal with the multi-recordings case.
This adds a lot of complexity everywhere (hashmaps, errors, etc), all the way to the end-users.

We should have a dedicated Rerun Archive format for these places where it is indeed a useful construct, and constrain RRD streams to a single recordings, always.
Most APIs would work at the RRD level, and therefore become simple both to use and implement, and users could easily get recordings in and out of rerun archives using dedicated APIs and/or CLI tools.