Enhance CommandDispatcher async methods to use CompletableFutures

This allows consumers of CommandDispatcher to implement non-blocking handling of dispatched commands.

The submitOnNode(...) method can just be modified to return the more specific return value. However, the submitOnCluster(...) will need a different method name (or signature).

The MessageDispatcher in JGroups 4 now returns CompletableFutures natively, which means we can easily support this without the addition of yet another thread pool.