Convert ThreadHelper.actor.h to standard coroutines

[tclinkenbeard-oai]

This PR converts ThreadHelper.actor.h to standard C++ coroutines and renames it to ThreadHelper.h.

To avoid performance regression, coro::DetachedCoroutine is added for onMainThreadVoid. It provides a fire-and-forget coroutine whose frame owns and destroys itself. This avoids allocating a result Future/SAV and omits cancellation bookkeeping because callers receive no handle.

Performance compared to actor implementation, using #13358 (negative is faster):

Batch	Plain enqueue	Error-member
1	-1.8%	+0.3%
4	+12.3%	+3.1%
16	-18.2%	-1.8%
64	-12.5%	+0.4%
256	-9.9%	+4.1%
1024	-13.0%	+3.9%*

The detached implementation reduces the initial error-member regression from 20–24% to within approximately 4%. Its coroutine frame also drops from the 128-byte to 96-byte allocation bucket.

[foundationdb-ci]

Result of foundationdb-pr-cluster-tests on Linux RHEL 9

• Commit ID: 347586c
• Duration 0:10:15
• Result: ❌ FAILED
• Error: Error while executing command: ninja -v -C build_output -j ${NPROC} all packages strip_targets. Reason: exit status 1
• Build Log terminal output (available for 30 days)
• Build Workspace zip file of the working directory (available for 30 days)
• Cluster Test Logs zip file of the test logs (available for 30 days)

[foundationdb-ci]

Result of foundationdb-pr on Linux RHEL 9

• Commit ID: 347586c
• Duration 0:10:35
• Result: ❌ FAILED
• Error: Error while executing command: ninja -v -C build_output -j ${NPROC} all packages strip_targets. Reason: exit status 1
• Build Log terminal output (available for 30 days)
• Build Workspace zip file of the working directory (available for 30 days)

[foundationdb-ci]

Result of foundationdb-pr-clang-ide on Linux RHEL 9

• Commit ID: 347586c
• Duration 0:19:17
• Result: ✅ SUCCEEDED
• Error: N/A
• Build Log terminal output (available for 30 days)
• Build Workspace zip file of the working directory (available for 30 days)

[foundationdb-ci]

Result of foundationdb-pr-macos-m1 on macOS 14.x

• Commit ID: 347586c
• Duration 0:32:36
• Result: ✅ SUCCEEDED
• Error: N/A
• Build Log terminal output (available for 30 days)
• Build Workspace zip file of the working directory (available for 30 days)

[foundationdb-ci]

Result of foundationdb-pr-clang-arm on Linux RHEL 9

• Commit ID: 347586c
• Duration 0:45:37
• Result: ✅ SUCCEEDED
• Error: N/A
• Build Log terminal output (available for 30 days)
• Build Workspace zip file of the working directory (available for 30 days)

[foundationdb-ci]

Result of foundationdb-pr-macos on macOS 14.x

• Commit ID: 347586c
• Duration 0:47:56
• Result: ✅ SUCCEEDED
• Error: N/A
• Build Log terminal output (available for 30 days)
• Build Workspace zip file of the working directory (available for 30 days)

[foundationdb-ci]

Result of foundationdb-pr-clang on Linux RHEL 9

• Commit ID: 347586c
• Duration 0:54:59
• Result: ✅ SUCCEEDED
• Error: N/A
• Build Log terminal output (available for 30 days)
• Build Workspace zip file of the working directory (available for 30 days)