Cocaine-framework-python: fixed tests.

* some tests have been broken after renaming
Fixed data loss bug on synchronous chain usage.

* if you created multiple chains via service invocation and lately invoke `get` method to get chain result synchronously, other chains are also activated (cause there is only thread local event loop). This led to the data loss. Now results are stored in chain object.
* errors are transmitted to `ChainItem.callback` method even if there is no next handler
* pending results are saved in `ChainItem` objects to prevent data loss on synchronous usage
* test has been written
Fixed extra memory consumption.
Merge branch 'master' of github.com:cocaine/cocaine-framework-python
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()])
 ```
Some documentation added.
Changelog bump.
RHEL version bump.