The WT_TIME_WINDOW_SET_START and WT_TIME_WINDOW_SET_STOP macros in src/include/timestamp_inline.h previously read txnid unconditionally before the write_prepare branch, then applied a WT_ACQUIRE_BARRIER() only in the aborted-transaction fallback path.

This change:

1. Moves the txnid read inside the write_prepare branch so the correct memory ordering is applied per code path.
2. In the prepare path, replaces the direct read + WT_ACQUIRE_BARRIER() with __wt_atomic_load_uint64_v_acquire, making acquire semantics explicit.
3. In the non-prepare path, uses __wt_atomic_load_uint64_v_relaxed since acquire ordering is not required there.
4. For the aborted-transaction fallback (upd_saved_txnid), uses __wt_atomic_load_uint64_relaxed instead of a plain read after a barrier.

This eliminates the ad-hoc barrier, makes the intended memory ordering self-documenting, and ensures only the paths that need acquire semantics pay for them.