Go version does not provide this, but the topic of sync.RWMutex
downgrading was raised up several times, at least

	https://github.com/golang/go/issues/4026
	https://github.com/golang/go/issues/23513
	https://groups.google.com/forum/#!topic/golang-nuts/MmIDUzl8HA0
	...

Atomic downgrading is often useful to avoid race window in between
Unlock and RLock and, as consequence, having the need to recheck things
after RLock.

We can put this complexity and logic into well-defined RWMutex primitive
instead of throwing it to be solved by every RWMutex user.