K2V #293

Specification:

Making a Gitea PR as this is easier to edit and to comment than on cryptpad

View spec at this URL

Implementation:

Testing:

Just a simple Python script that does some requests to check visually that things are going right (does not contain parsing of results or assertions on returned values)
Actual tests:
- Adapt testing framework
- Simple test with InsertItem + ReadItem
- Test with several Insert/Read/DeleteItem + ReadIndex
- Test all combinations of return formats for ReadItem
- Test with ReadBatch, InsertBatch, DeleteBatch
- Test with PollItem
- Test error codes
Fix most broken stuff
- test PollItem broken randomly
- when invalid causality tokens are given, errors should be 4xx not 5xx

Improvements:

quick review of k2v spec. Overall looks good, see comments for details

 - **Internal server errors on updates do not mean that the update isn't stored.**
   K2V will return an internal server error when it cannot reach a quorum of nodes on
   which to save an updated value. However the value may still be stored on just one
   node, which whill then propagate it to other nodes asynchronously via anti-entropy.

     prefix: null,
     start: null,
     end: null,
     singleItem: false,

 - K2V triplets exist separately from S3 objects. K2V triples don't exist for
   the S3 API, and S3 objects don't exist for the K2V API.
 - Values stored for triples have associated causality information, that enables

 - a partition key (`pk`), an utf8 string that defines in what partition the triple is
   stored; triples in different partitions cannot be listed together, they must
   be the object of different ReadItem or ReadBatch queries