srw_mutex::wait_and_lock(): In the spin loop, we will try to poll
for non-conflicting lock word state by reads, avoiding any writes.
We invoke explicit std::atomic_thread_fence(std::memory_order_acquire)
before returning. The individual operations on the lock word
can use memory_order_relaxed.

srw_mutex::lock: Document that the value for a single writer is
HOLDER+1 instead of HOLDER.

srw_mutex::wr_lock_try(), srw_mutex::wr_unlock(): Adjust the value
of the lock word of a single writer from HOLDER to HOLDER+1.