[General] Allow specifying extra log attributes.

Can now create logs with extra attributes, not only "source" one.
[ASIO] Fixed a race in engine's destructor.

Store the engine's id on log creation, instead of querying it every time a new log message is being
created, including logging of dying sessions while engine's destructor is running.
[ASIO] Less indirection in Engine's Chamber.
[ASIO] Destroy upstreams together with dispatches.
[ASIO] Drop an unnecessary const specifier.
[RPC] Allow rvalue references in RPC method calls.
[Logging] Use rvalue references for SUPERSPEED!
[General] Prettifying.
Merge branch 'master' of https://github.com/cocaine/cocaine-core