Start chunk size changed.
Drop saving exception context.

* because it doesn't work properly anyway.
For optimization __debug__ statement is back!
More verbose FutureResult string representation.
Skip broken test.
Log messages revamped.
PreparedFuture is firing callback immediately now.

* less event loop usage.
No more excess event loop usage in chain items.

* this significantly boost framework performance.
Naming over GeneratorFutureMock.

* also it doesn't use event loop anymore.
* neither ChainItem does.
ConcurrentWorker refactored.

* naming
And naming again.
Code cleanup.
Drop unnecessary collection usage.
More concrete condition statement in ChainItem.

* also more verbose log output when exception reached
  end of the chain uncaught.
Documentation update.
More docs!
Move cocaine log handled into `handlers` module.

* also renamed it into `CocaineHandler`
Drop `Response:` words in call action.