File system maintenance service and its btrfs handle

[happyaron]

This PR includes a new file system maintenance service and its btrfs handle, suggested to complete the mitigation in #235.

Two files are added:

• extras/bin/fs-service: Dumb service wrapper to get file system maintenance tasks done properly under snappy constrains where timers are not always available. It schedules different tasks under defined situations with proper tracking.
• extras/bin/btrfs-maint: Handle of maintaining btrfs file system, scheduled by fs-service. It takes care of a strategy of improving the health of btrfs's on-disk status by issuing sync, balance and scrub operations periodically with constraints. IO activities are heavily throttled according to the system load and IO status, even though it needs a better implementation to take care of the IO information.