nilfs-utils: add support for tracking live blocks

This patch adds a new feature flag NILFS_FEATURE_COMPAT_TRACK_LIVE_BLKS,
which allows the user to enable and disable the tracking of live blocks.

Signed-off-by: Andreas Rohner <andreas.rohner@gmx.net>
nilfs-utils: add support for counting of snapshots

TEMP

Signed-off-by: Andreas Rohner <andreas.rohner@gmx.net>
nilfs-utils: add support for greedy/cost-benefit policies

TEMP

Signed-off-by: Andreas Rohner <andreas.rohner@gmx.net>