BFQ and related stuff on disk scheduling

BFQ-v3 sports the following new features (see the changelog in README.BFQ for more details):

According to our tests on 2.6.39 and 3.0, none of these features is paid with a loss of aggregate throughput with respect to BFQ-v2. As usual, there is still room for improvement, because, e.g., interactive applications happen occasionally to experience a quite higher latency than the average.

The patchset and any incremental patch can be downloaded here.