MERGE3: merging in large clusters

MERGE2 has the issue that concurrent merges are frequent with large clusters. MERGE3 will change this and works as follows:

• Every node sends a multicast with its address, logical name, physical address and ViewId every N seconds
• Every node collects this information and - every N * 1.5 seconds - determines (a) whether we have different ViewIds and (if so) (b) determines the merge leader from the collected info
• The merge leader then collects the different Views from the merge participants, and sends a MERGE event up the stack, so that GMS can start the merge

The benefits of MERGE3 are:

• Good for large clusters, geared toward UDP based stacks
• Only 1 merge in the cluster at any time
• The collection of information to see whether a merge is needed is spaced out over a longer time period (N secs), compared to MERGE2, which uses Discovery.findAllViews(), which consists of a multicast and N unicasts. This might cause temporary congestion and packet loss
• On a merge, the members already have the logical-physical address mapping (unlike MERGE2)