Lower debugging in sal-clustering-commons

Since this component is under controller.cluster.datastore,
it floods logs with serialization details when we really just
want to understand what the data store is doing. Lower the logs
to TRACE.

Change-Id: Ib9a4735bc439a726452fb98fb5f79d2b7dea2047
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>