FileUploader first commit.

Collects files, renames them upon size and age constraints, tests the above.

Lots of ```TODO(dkorolev)```. And time to discuss export events in more detail.
Let's chat offline:

1) whether we need non-POSIX support now (I guess no?)
2) whether we stick with ```int  fd;```, ```FILE* f;``` or ```std::ofstream;```:? (I'd assume ```FILE* f;```, but not sure it's what we've agreed upon yesterday.)