Rotator changes.

Rotator structs are classes now. Absent includes added. Also minor wording.
File sink refactoring.

* There is no `null_rotator_t` class. Using template specialization instead of it.
* Reduced constructor number for `file::config_t`.
* Default values for rotator config.
* File sink has been refactored. The simplest file sink’s functionality has been splitted into writer and flusher.
* True sink name parser has been excluded from `config_traits`. Now it is free function, but not for long time.
Major renaming done.
Removed trivial (but unused) constructor for file sink.
Moving constant from function definition.
Rotator config now has initialization constructor with default values.
Rotator now depends on backend type (for universality).
Added file sink specialization with rotation.
Rotator template parameter is now template itself.

NoRotation tag added instead of `void`.
First implementation of file rotation.

File backend can now close, rename files and return its filename.
Microrefactoring.
Rotation improved. File backends now can check file existence.
File sink path is immutable now.
Improvements of rotation algorithm. Some tests added.