Use monotonic time to compute intervals in broker's submonitors

The submonitors stopped reporting data when the system time
on a host was moved backwards.

Change-Id: I775ec7b9f11b253d06c45eb01a765123088944aa
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1036683
Signed-off-by: Martin Sivak <mars@montik.net>