domainMonitor: Separate change detection from lastCheck value

Since commit 45f3037ca1e, lastCheck value is initialized to special
NOT_CHECKED_YET value, and used for detecting the first monitor status change.
This change caused repoStats to return high lastCheck value until the first
monitor check is done, causing host activation to fail, and host becoming
non-operational.

Previously, lastCheck value was initialized to monitor thread creation time,
and repoStats was returning valid lastCheck value even before the first domain
check was finished. While somewhat incorrect, this behavior is required for
engine monitoring logic.

This patch restore the previous lastCheck semantics and use a new
firstChange flag for detecting the first status change.

Change-Id: I8e0df2ee0333354146b5d9429fd83a4f914ff751
Relates-To: https://bugzilla.redhat.com/1003588
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21878
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Liron Ar <laravot@redhat.com>
Reviewed-by: Ayal Baron <abaron@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>