Tuner refactoring

Created by: valer1435

Now tuner uses metrics that not compatible with composer metrics. Also it has a monolith structure.

What should be done:

Union metrics for composer and tuner
Make tuner more flexible using decomposition
Implement self._objective as separate module
Refactor view of model parameters. Now they become too complicated to store them into dict

UPD (from @gkirgizov) Additional subtasks:

HyperOpt.get_metric_value seems to duplicate logic of PipelineObjectiveEvaluate that's concerned with how to validate the model -- so it's better to use it instead.
Abstract Tuner to arbitrary Graphs. The only place where the specifics of Pipeline are required -- is for evaluation of objective. But, I think, currently this can be encapsualted with Objective/ObjectiveEvaluate classes.

Regarding 1st and 3rd points by Valera, possibly Objective class is useful here.