Iterators flow control and python binding improvements. 

* Added ability to pause/continue/cancel iterators by id. Now elliptics keeps track of all iterators.
* Python binding improvements around iterators. 
* Now we not locking iteration key while iterator is running.
* Misc build improvements. Mostly removed compiler warnings.
* Couple of unrelated bugfixes 

Should be safe. Especially considering that most of patch touches only iterator's code.