Relax MeteredExecutor state update ordering

[namtran1812]

Summary

Relax the successful MeteredExecutorImpl::modifyState() compare-exchange from memory_order_seq_cst to memory_order_relaxed.

state_ is a packed bookkeeping word containing the pending-task count, pause state, and number of workers in the wrapped executor queue. Its updates require atomicity and per-object modification ordering, but do not publish task contents or synchronize unrelated memory.

Task publication is handled separately by UMPMCQueue, which provides its own synchronization between enqueue and dequeue.

pause() already updates the same state_ word with a relaxed atomic RMW.

Details

modifyState() has three callers:

• add(): enqueues the task first, then updates pending/worker counts
• resume(): clears the paused bit and reserves workers to schedule
• worker(): decrements pending/worker counts before dequeuing the task

None of these uses the state_ CAS as a cross-object publication mechanism.

The initial load and failed CAS in modifyState() are already relaxed, so this removes the remaining unnecessary sequentially-consistent operation from the state-update path.

Related to #2656.

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