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.
I was thinking of a dedicated ```struct ClientFileStorageLogic``` with all the typedef, defaulted to the right ones?

```FILE_APPEND_POLICY``` may go, but I was about to serialize text line-by-line in the test, therefore, instead of simply appending new messages, an extra newline will be inserted.

```FILE_SYSTEM``` may go, but then I won't be able to test very large files, not to mention the need to support file timestamp setting in the test.

I'm all in for keeping things simple, but these guys look important to me here.