More key-value store backends.

Nobody has claimed this yet.

RefineDB's key-value store backends need to be transactional and support all operations in the `KvTransaction` trait:

```rust
pub trait KvTransaction: Send + Sync {
async fn get(&self, key: &[u8]) -> Result>>;
async fn put(&self, key: &[u8], value: &[u8]) -> Result<()>;
async fn delete(&self, key: &[u8]) -> Result<()>;
async fn delete_range(&self, start: &[u8], end: &[u8]) -> Result<()>;
async fn scan_keys(&self, start: &[u8], end: &[u8]) -> Result>;
async fn commit(self: Box) -> Result<(), KvError>;
}
```

Currently supported key-value store backends:

- [x] FoundationDB
- [x] SQLite
- [ ] TiKV: Missing efficient transactional range deletion. ([Discussion](https://tikv-wg.slack.com/archives/C01H8ULVANR/p1624950773020000))
- [ ] RocksDB: Missing transaction support in existing Rust bindings.

Start with the KvTransaction trait shown in the issue and compare the existing FoundationDB and SQLite backends with the TiKV and RocksDB limitations described. Determine whether each candidate can provide all listed operations transactionally, including range deletion and commit. Done means an additional backend implements the complete trait without the stated transactional limitation.

Written by the indexing model from the issue text.