Deprecate default-stack JGroups subsystem attribute

The original purpose of the default-stack attribute was to provide a default ChannelFactory implementation. However, with the introduction of fork channels, there are now 2 candidate for a default ChannelFactory:
1. The JChannelFactory of the default stack.
2. The ForkChannelFactory of the default channel.

#2 is the better option - and is what Infinispan transports use by default.

This would require that the stack attribute of a channel is required (currently, it is optional and defaults to the default-stack).