fix: rate_runner

[alwayslove2013]

Background:

• The rate_runner creates a thread pool within a single process to handle submit_by_rate.
• In contrast, the read_write_runner spawns multiple processes (equal to concurrency) for search tests.

==> It imposes significant scheduling pressure on the vdbbench machine. As a result, submit_by_rate often takes longer than 1s to complete.

In this PR:

• Increased the warning threshold for submit_by_rate.
• Modified check_and_send_signal to avoid waiting.
• By the way, suppressed warning logs for minor task insertions to reduce unnecessary noise.

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: alwayslove2013, XuanYang-cn

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