Exchanges any read-side critical sections associated with lhs and rhs.
*this is associated with the read-side critical section (if any) associated with other prior to the invocation. other is associated with the read-side critical section (if any) associated with *this prior to the invocation.
If either of *this or other have an associated read-side critical section prior to the invocation, those read-side critical sections must have started on the same thread as the invocation of swap.