continuous-integration/drone/push Build is passingDetails
**Specification:**
View spec at [this URL](https://git.deuxfleurs.fr/Deuxfleurs/garage/src/branch/k2v/doc/drafts/k2v-spec.md)
- [x] Specify the structure of K2V triples
- [x] Specify the DVVS format used for causality detection
- [x] Specify the K2V index (just a counter of number of values per partition key)
- [x] Specify single-item endpoints: ReadItem, InsertItem, DeleteItem
- [x] Specify index endpoint: ReadIndex
- [x] Specify multi-item endpoints: InsertBatch, ReadBatch, DeleteBatch
- [x] Move to JSON objects instead of tuples
- [x] Specify endpoints for polling for updates on single values (PollItem)
**Implementation:**
- [x] Table for K2V items, causal contexts
- [x] Indexing mechanism and table for K2V index
- [x] Make API handlers a bit more generic
- [x] K2V API endpoint
- [x] K2V API router
- [x] ReadItem
- [x] InsertItem
- [x] DeleteItem
- [x] PollItem
- [x] ReadIndex
- [x] InsertBatch
- [x] ReadBatch
- [x] DeleteBatch
**Testing:**
- [x] 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)
- [x] Actual tests:
- [x] Adapt testing framework
- [x] Simple test with InsertItem + ReadItem
- [x] Test with several Insert/Read/DeleteItem + ReadIndex
- [x] Test all combinations of return formats for ReadItem
- [x] Test with ReadBatch, InsertBatch, DeleteBatch
- [x] Test with PollItem
- [x] Test error codes
- [ ] Fix most broken stuff
- [x] test PollItem broken randomly
- [x] when invalid causality tokens are given, errors should be 4xx not 5xx
**Improvements:**
- [x] Descending range queries
- [x] Specify
- [x] Implement
- [x] Add test
- [x] Batch updates to index counter
- [x] Put K2V behind `k2v` feature flag
Co-authored-by: Alex Auvolat <alex@adnab.me>
Reviewed-on: #293
Co-authored-by: Alex <alex@adnab.me>
Co-committed-by: Alex <alex@adnab.me>
