Improve the support for ScyllaDb. (#3106)

## Motivation

The support of ScyllaDb is critical as it is our main database for
operations.

## Proposal

* Switch from `Logged` to `Unlogged` for the batch types. This is fine
because our transactions are on a single root_key and a single partition
key.
* Make the `expand_colliding_prefix_deletions` a little faster by not
constructing the BTreeSet if it does not need to be used.
* Switch from version 0.12.0 to version 0.15.1
* Switch from Query to Prepared statements.

## Test Plan

(A) The CI

(B) The runtimes are obtained by the benchmarks:
- `store_scylladb_write_batch`: 28.32 ms on this PR vs 31.607 ms
- `store_scylladb_find_key_values_by_prefix`: 15.025 ms vs 16.426 ms
- `store_scylladb_find_keys_by_prefix`: 1.5997 ms vs 3.1336 ms
- `store_scylladb_contains_keys`: 4.52 ms vs 4.46 ms
- `store_scylladb_contains_key`: 94.82 ms vs 248 ms
- `store_scylladb_read_multi_values_bytes`: 357 mus vs 354 mus.
- `store_scylladb_read_value_bytes`: 220 mus vs 208 mus.

## Release Plan

The improvement is of interest to TestNet / DevNet users.

## Links

The Scylla documentation: https://docs.rs/scylla/0.15.1/scylla/

Author: MathieuDutSik