Freeze data at rest format

[alexsnaps]

We want to have a stable and performant format to store counters on disk (whether Redis or whatever). It should probably have the following characteristics:

• [ ] binary format, i.e. compact. e.g. postcard
• [ ] have a version encoded in it, so it can be (lazily) changed moving forward
• [ ] store all counters for each time window of a given RLP in one entry

[alexsnaps]

So we can't do any of this... but there is two things that remain to solve:

• Ensure partitioning prefix for Redis is respected in the new v2 format for id'ed limits
• Set some marker to store the format "mode" so we can get our of this mess one day