Based on PR #44 by Johannes Schirm <https://github.com/Marth68>.
The code is pretty much the same, but with usn used instead of
mtime for determining which media files have changed since last
sync.
Relevant excerpt from #44:
> The server was always sending metadata for all files in the
> collection as a response to the mediaChanges operation. Because
> AnkiDroid then calculates checksums for all these files (just to
> notice that they actually haven't changed), it took larger
> collections very long to sync after every media change. With this
> fix, only the number of files indicated by usn - lastUsn (and
> mtime) are considered for each sync.
Fixes#26, closes#44.
e8f274ee84