Provide from debugging info in Actor and Producer toString

[elizarov]

Inspired by this question: https://stackoverflow.com/questions/52829802/how-can-i-know-how-many-messages-are-waiting-for-actor-to-process?noredirect=1#comment92593433_52829802

The proposal is to add channel's debug info to Actor and Producer debug toString() implementation. For buffered channels it might be important to see the number of messages that are waiting to be processed. Take this into account when working on complex actors #87

[oshai]

maybe also as other properties than the to strings so it can be added to metrics etc'