Currently, we increase the cluster serial number for instance additions,
removals and renames. This is conforming with the REST paradigm, however
it means that for each of these operations, we need to push ssconf
updates to all nodes.

In order to support future cases with reduced set of master-eligible
nodes, we want to reduce the ssconf pushes (which need to be to all
nodes). This patch changes the meaning for the cluster serial number so
that it doesn't track instance operations at all.

This means that addition of an instance can fail due to concurrent
additions, even if the cluster serial has not changed. It slightly
breaks the REST paradigm, but IMHO it's better for actual usage.

Reviewed-by: ultrotter