app/vlstorage: extend top-N optimization to support sort_direction=asc

[cuongleqq]

Summary:

This PR adds sort_direction query arg to /select/logsql/query. When limit=N is specified, passing sort_direction=asc returns the N oldest matching log entries.

Before, limit=N always used a binary-search optimization on the time range to efficiently return the last N logs without a full scan. This PR generalizes that optimization to support both directions.

• It changes the name of most functions related to lastN, to timeSortedN, to mean "both directions"
• It uses a struct timeSortedRangeOps to generalize the operation on the time range
• Most of other changes are just passing isDesc around to specify the sort directions

Closes #924

[cubic-dev-ai]

No issues found across 59 files

[cuongleqq]

@func25 thanks for the comments.

I changed the asc and desc struct to 2 helpers. I think it simplifies the code flow.

I also changed everything "last n" to "time sorted n results". I actually struggle with the naming for this, but I don't have a better name 🙃.