Fix Fiber leaks if Suspension never resumed

[xjaja]

Have a circular reference between the weak map of key and value.

It splits the circular reference of key and value.

Now, the Fiber with Suspension object, so if the object is not keep, garbage collected will occur at the next Fiber suspend .

related #52 (comment).

[kelunik]

@trowski Thoughts?

[trowski]

Wrapping the suspension into a WeakReference effectively removes reuse of Suspension objects. Should we instead drop this cache entirely?

If it useful for debugging (personally I haven't really found it to be), we could keep an array of WeakReferences like this but only if assertions are enabled.

[trowski]

Closing in favor of #78.

[kelunik]

@trowski I favor this one over the other, because it has less BC impact.

[trowski]

What sort of BC impact are you seeing? Driver::getSuspension() will always return a new instance in this PR unless the user happened to retain a reference to the prior suspension (which they really shouldn't if not suspended). The modified APIs are all internal, so the only change to the public API is that a new Suspension instance is always returned.