Add persisting for failed requests

[igorrKurr]

For now we have requests running in queue for retryCount times when failed.
But it is better to add some persistence for such failed reqs, that wasn't successful even after multiple times in queue.
If we'll do so, then consistency problem may appear.
To solve it without providing any additional uuid, etc. we can use git-like approach.
For example, we can provide some options in config like strategy: 'use-theirs/use-our' that will behave similar to git merge strategies.
To do so we can put all responsibility for changing data to both user and server.