### Describe Your Changes
Allow disabling the per-day index using the `-disablePerDayIndex` flag.
This should significantly improve the ingestion rate and decrease the
disk space usage for the use cases that assume small or no churn rate.
See the docs added to `docs/README.md` for details.
Both improvements are due to no data written to the per-day index.
Benchmark results:
```shell
rm -Rf ./lib/storage/Benchmark*; go test ./lib/storage -run=NONE -bench=BenchmarkStorageInsertWithAndWithoutPerDayIndex --loggerLevel=ERROR
goos: linux
goarch: amd64
pkg: github.com/VictoriaMetrics/VictoriaMetrics/lib/storage
cpu: 13th Gen Intel(R) Core(TM) i7-1355U
BenchmarkStorageInsertWithAndWithoutPerDayIndex/HighChurnRate/perDayIndexes-12 1 3850268120 ns/op 39.56 data-MiB 28.20 indexdb-MiB 259722 rows/s
BenchmarkStorageInsertWithAndWithoutPerDayIndex/HighChurnRate/noPerDayIndexes-12 1 2916865725 ns/op 39.57 data-MiB 25.73 indexdb-MiB 342834 rows/s
BenchmarkStorageInsertWithAndWithoutPerDayIndex/NoChurnRate/perDayIndexes-12 1 2218073474 ns/op 9.772 data-MiB 13.73 indexdb-MiB 450842 rows/s
BenchmarkStorageInsertWithAndWithoutPerDayIndex/NoChurnRate/noPerDayIndexes-12 1 1295140898 ns/op 9.771 data-MiB 0.3566 indexdb-MiB 772119 rows/s
PASS
ok github.com/VictoriaMetrics/VictoriaMetrics/lib/storage 11.421s
```
Signed-off-by: Artem Fetishev <wwctrsrx@gmail.com>
Signed-off-by: Artem Fetishev <rtm@victoriametrics.com>
Co-authored-by: Roman Khavronenko <hagen1778@gmail.com>
