Operating with common std::iostream instead of only file streams

[eugenyk]

FFStream inheritance changed from std::fstream to std::iostream with backward compatibility, that leads to a couple of features:

• possibility to use streams not connected to filesystem in cases when information comes from network
• writing unit tests without work with filesystem, simple std::stringstream can be used

[masamitsutech]

This pull request will be reviewed for the next stable release.

This pull request will be closed if the change is accepted and merged into the toolkit. If any issues are found, we will report those here to be resolved.