Don't run the "Date" header timer every second all the time

[Jarred-Sumner]

What does this PR do?

Only reschedule the Date header while there are in-flight incoming HTTP requests.

Update the Date header if, at the time we reschedule it, it is now stale.

Goal: don't wake up Bun's process on every second when we're idly doing nothing.

Metric	this branch	main
task-clock	35.24 msec 🟢	102.79 msec
context-switches	619 🟢	1,699
cpu-migrations	11 🟢	35
page-faults	2,173	2,174
cpu_atom/instructions	109,904,685 (1.76 insn/cycle) 🟢	67,880,002 (0.55 insn/cycle)
cpu_core/instructions	87,183,124 (1.07 insn/cycle) 🟢	32,939,500 (0.44 insn/cycle)
cpu_atom/cycles	62,527,125 (1.774 GHz) 🔻	122,448,620 (1.191 GHz)
cpu_core/cycles	81,651,366 (2.317 GHz) 🟢	75,584,111 (0.735 GHz)
cpu_atom/branches	9,632,460 (273.338 M/sec) 🔻	12,119,616 (117.909 M/sec)
cpu_core/branches	17,417,756 (494.259 M/sec) 🟢	6,901,859 (67.147 M/sec)
cpu_atom/branch-misses	192,013 (1.99%) 🟢	1,735,446 (14.32%)
cpu_core/branch-misses	473,567 (2.72%) 🟢	499,907 (7.24%)
TopdownL1 (cpu_core)	31.4% backend_bound 11.7% bad_speculation 36.0% frontend_bound 🔻 20.9% retiring 34.1% bad_speculation 41.9% retiring 0.0% backend_bound 24.0% frontend_bound 🔻	21.3% backend_bound 9.6% bad_speculation 56.2% frontend_bound 12.9% retiring -20.0% bad_speculation 55.2% retiring 26.2% backend_bound 38.6% frontend_bound
time elapsed	1000.0219 s	1000.0107 s
user time	—	0.042667 s
sys time	—	0.060309 s

How did you verify your code works?

Added a test

[robobun]

^{Updated 2:36 AM PT - Aug 14th, 2025}

❌ @autofix-ci[bot], your commit 43d4ccd has 6 failures in Build #22923:

• test/js/bun/http/req-url-leak.test.ts - 1 failing on 🍎 13 aarch64
• test/integration/mysql2/mysql2.test.ts - timeout on 🍎 14 aarch64
• test/bake/dev/ssg-pages-router.test.ts - 1 failing on 🍎 13 aarch64
• test/js/sql/sql.test.ts - 1 failing on 🐧 24.04 x64
• test/js/sql/sql.test.ts - 1 failing on 🐧 25.04 x64
• test/js/sql/sql.test.ts - 1 failing on 🐧 24.04 x64-baseline
• test/js/sql/sql.test.ts - 1 failing on 🐧 25.04 x64-baseline
• test/js/node/test/parallel/test-child-process-spawnsync-shell.js - timeout on 🪟 2019 x64-baseline
• test/js/node/test/parallel/test-child-process-spawnsync-shell.js - timeout on 🪟 2019 x64
• test/js/bun/shell/pipeline_stack.test.ts - 1 failing on 🪟 2019 x64
• test/js/bun/shell/pipeline_stack.test.ts - 1 failing on 🪟 2019 x64-baseline

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🧪   To try this PR locally:

bunx bun-pr 21850

That installs a local version of the PR into your bun-21850 executable, so you can run:

bun-21850 --bun