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.
Non-standard appender might add something else. As the message gets longer (say, it adds device info, which might be long), it may affect fine finalization policy (rename sooner) and file purge policy (delete older files earlier).