storaged: clean up disk stats code

Merged disk_stats_publisher into disk_stats_monitor class.

Moved proc file check to disk_stats class instead of doing so in
storaged, same for uid_io.

Added is_zero function to check zero disk_stats.

Added operators to compute disk_stats.

Change-Id: I277ae51f6fe18464f92e45c03471343784e67093
12 files changed