feat: multi-thread reads and batched writes

[losfair]

Write batching and read multi-threading improves throughput by 5-18x, depending on workload type.

Benchmark with:

denokv --sqlite-path denokv-test.sqlite serve --access-token public-token --num-workers 8 --atomic-write-batch-timeout-ms 50
deno run -A --unstable ./scripts/benchmark.ts --path http://127.0.0.1:4512 --read-concurrency 128 --write-concurrency 64 --size 10000 --consistency-check-interval 10

Before:

Inserted 10000 entries in 18661ms
NewRds:     1831/s, NewWrs:      472/s, NewCfs:        7/s,

After:

Inserted 10000 entries in 1317ms
NewRds:    34573/s, NewWrs:     2285/s, NewCfs:       38/s

[littledivy]

Great improvements, nice work.