Add user API to handle merging HttpSession attributes/metadata on partition merge

In the event of a detected network partition, the distributed web session manager's Infinispan cache behaves in one of 3 ways:

1. Disallow both reads and writes from/to a web session in all partitions
2. Allow reads from a web session in all partitions, but only allow writes to a web session if the partition contains all data owners.
3. Allow both reads and writes to/from a web session in all partitions

For customers who opt for #3 (i.e. maximum availability), in the event that a web session is updated in both partitions, they need be able to provide a mechanism to resolve conflicts when the partitions eventually merge.

Something like:

public interface SessionAttributeMergePolicy {
    Object merge(String attributeName, Object primaryPartitionAttributeValue, Iterable<Object> minorityPartitionAttributeValues); 
}