Enable query request queuing by default when total laning is turned on.

[abhishekagarwal87]

This PR enables the flag by default to queue excess query requests in the jetty queue. Still keeping the flag so that it can be turned off if necessary. But the flag will be removed in the future.

Release notes

• When query scheduler threads are less than server HTTP threads, total laning becomes on. This lets reserve some HTTP threads for non-query requests such as health checks. The total laning previously would reject any query request that exceeds the lane capacity. But now, those excess requests will instead be queued with a timeout equal to MIN(Integer.MAX_VALUE, druid.server.http.maxQueryTimeout)

if value of druid.server.http.maxQueryTimeout is negative, then the excess requests will be queued forever.

This PR has:

• been self-reviewed.
• been tested in a test Druid cluster.

[LakshSingla]

Perhaps we should point it out in the developer release notes in the next release.

[gianm]

The docs for druid.query.scheduler.numThreads should be updated too, as the out of box config will no longer result in HTTP 429 when the scheduler limit is exceeded.

[abhishekagarwal87]

Updated the docs.

[maytasm]

Is this doc incorrect https://github.com/apache/druid/blob/master/docs/configuration/index.md?plain=1#L1777
It says "Requests in excess of the capacity are discarded with an HTTP 429 status code."

[abhishekagarwal87]

No. It's correct. This PR queues requests when the total lane is at capacity but any other lane will still see the excess request being rejected.