CacheRegistry is missing entries (e.g. client mappings) following a merge after a cluster split

One of the manifestation of the issue:

1. start 2 nodes with SLSB with TUNNEL transport
2. start both nodes creating 2 clusters (or partitions)
3. start ejb client
4. start GossipRouter and wait for merge
5. ejb client keeps talking only to known node; never receives a topology update

This is because org.wildfly.clustering.server.registry.CacheRegistry#topologyChanged does not handle cluster merges and thus all entries from a given partition will be lost forever.

The effects are especially missing client mappings and broken session stickiness.