util: Infrastructure for computing recent averages

This module computes the average of a set of values within a time
window, keeping also track of the minimum and maximum values.

In order to produce more accurate results it works internally by
creating two time windows of the same period, offsetted by half of
that period. Values are accounted on both windows and the data is
always returned from the oldest one.

[Add missing util/replay.o to test-timed-average dependencies to fix the
build.
--Stefan]

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 201b09c21bbc9c329779d2b2365ee2b9c80dceeb.1446044837.git.berto@igalia.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
