First implementation of K2V (#293)

**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: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/293
Co-authored-by: Alex <alex@adnab.me>
Co-committed-by: Alex <alex@adnab.me>

Cargo.lock

@@ -29,6 +29,16 @@ version = "0.5.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "23b62fc65de8e4e7f52534fb52b0f3ed04746ae267519eef2a83941e8085068b"
 
+[[package]]
+name = "assert-json-diff"
+version = "2.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "50f1c3703dd33532d7f0ca049168930e9099ecac238e23cf932f3a69c42f06da"
+dependencies = [
+ "serde",
+ "serde_json",
+]
+
 [[package]]
 name = "async-stream"
 version = "0.3.3"
@@ -821,8 +831,10 @@ dependencies = [
 name = "garage"
 version = "0.7.0"
 dependencies = [
+ "assert-json-diff",
  "async-trait",
  "aws-sdk-s3",
+ "base64",
  "bytes 1.1.0",
  "chrono",
  "futures",
@@ -846,6 +858,7 @@ dependencies = [
  "rmp-serde 0.15.5",
  "serde",
  "serde_bytes",
+ "serde_json",
  "sha2",
  "sled",
  "static_init",
@@ -876,6 +889,7 @@ dependencies = [
 name = "garage_api"
 version = "0.7.0"
 dependencies = [
+ "async-trait",
  "base64",
  "bytes 1.1.0",
  "chrono",
@@ -886,6 +900,7 @@ dependencies = [
  "futures-util",
  "garage_block",
  "garage_model 0.7.0",
+ "garage_rpc 0.7.0",
  "garage_table 0.7.0",
  "garage_util 0.7.0",
  "hex",
@@ -966,6 +981,8 @@ version = "0.7.0"
 dependencies = [
  "arc-swap",
  "async-trait",
+ "base64",
+ "blake2",
  "err-derive 0.3.1",
  "futures",
  "futures-util",