Backport ROW scheduler from CAF 3.4

The ROW scheduling algorithm will be used in mobile devices as default
block layer IO scheduling algorithm. ROW stands for "READ Over WRITE"
which is the main requests dispatch policy of this algorithm.

The ROW IO scheduler was developed with the mobile devices needs in
mind. In mobile devices we favor user experience upon everything else,
thus we want to give READ IO requests as much priority as possible.
The main idea of the ROW scheduling policy is:
If there are READ requests in pipe - dispatch them but don't starve
the WRITE requests too much.