Drop class loggers in favor to global thread-safe logger.

* module cocaine.futures.chain
Replace ioloop singleton usage by its thread local brother.

* also some renaming done
Some classes are now thread-safe.

* also docs fixed
Example are back.
Performance boost over chain API.

* approximately by 16% if chain API is used.
* now representation of values is calculated deferredly while constructing log message.
Introducing asynchronous future grouping.

 * now you can yield list of futures, waiting for control returns after all of them completely execute.
 * tests has been written

 Example:
 ```
 s1 = Service('s1')
 s2 = Service('s2')

 def func():
    r1, r2 = yield chain.All([s1.execute(), s2.execute()])
 ```