Data Centre - don't trigger cluster panic when node is merging

It was discovered that often, after false eviction of cluster node (f.e. b/c of long running GC pause), node panics when hazelcast actually tries to join that problematic node back to cluster. We need to disable panic when node received MERGING event from the cluster.
Investigation needs to be done when we need to re-set MERGING flag and allow panic again (on MERGED / timeout / etc)

NB: This has been superseded by CONF-42954.