[WIP] Race conditions handling

[rtrv]

Just a concept for #94, I want to discuss it. Current solution is following:

• Some action gets a session and also gets it as 'initial_session_state'
• When it sets the session back, it compares its initial state with current session state
• If there is any difference, session data from parameters will be merged with current session state

Current action data have higher priority in this case. I think, if everybody has no problems with this concept, we could discuss of tests or another methods structure maybe.

This behaviour is optional by ':handle_race_conditions' boolean key in configuration.