BUG-8618: add threshold crossing debugs

We are observing messages about sync status changing on the order
of 10s a second (14ms between messages). This looks awfully like
inter-node latency, hence it needs to be tuneable.

We do not have an understading of what sort of jumps are we talking
about, so add logging to the source of this events at debug, so these
can be diagnosed.

Change-Id: I9e2d78629f8808914cdb664cb28afcd47a55ee80
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 97875ef2635a536c0663750503ea6e64486d2fe6)