sync: useful Debug impl for RwLock

[FHTMitchell]

Motivation

Currently the std::fmt::Debug impl for tokio::sync::RwLock is just what #[derive(Debug)] provides and looks like

RwLock { mr: 536870911, s: Semaphore { permits: 536870911 }, c: UnsafeCell { .. } }

which isn't very useful. Instead we can show the data if no thread currently holds the write lock. This matches what is done in std::sync::RwLock and in tokio::sync::Mutex.

Solution

The updated code is basically a 1 to 1 mapping from the tokio Mutex implementation.

[Darksonn]

Looks good to me.