Changes

This is a working proposal for the backend storage architecture of PiCL server. It tries to take some of the good bits from the Firefox Sync backend, add in some lessons learned from running that in the field, simplify things a little, and make some adjustments towards stronger durability. It is far from final. All feedback welcome!

* Scale to billions of users. Quickly. Easily.

* Each user's data is completely independent, there's no need for queries that cross multiple user accounts.

* Each user account gets an opaque, immutable user id.