Refactor Unix file manager to use Java NIO.2 API

[DoktorZhor]

Java NIO.2 API offers cleaner way to handle files, which should simplify the Unix file manager implementation.

https://docs.oracle.com/javase/8/docs/technotes/guides/io/index.html

[DoktorZhor]

Redundant, as the IO methods which we're supposed to benefit from this the most, were in the end implemented using NIO.2 from the start.