lib: Proportions with flexible period

Implement code computing proportions of events of different type (like code in
lib/proportions.c) but allowing periods to have different lengths. This allows
us to have aging periods of fixed wallclock time which gives better proportion
estimates given the hugely varying throughput of different devices - previous
measuring of aging period by number of events has the problem that a reasonable
period length for a system with low-end USB stick is not a reasonable period
length for a system with high-end storage array resulting either in too slow
proportion updates or too fluctuating proportion updates.

Change-Id: I63575f8ce47f6a05757dcfa8dae8f8617c12a4bf
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Luis Cruz <ljc2491@gmail.com>

Conflicts:
	lib/Makefile