Message: add a flag to discard own multicast

If we have a group consisting of members

and A wants to multicast to the group, it will receive its own message too. To avoid this, A can set option LOCAL on its channel to false, meaning that all messages sent by A will get discarded by A's channel.

The problem here is that those messages need to get handled nevertheless (e.g. by NAKACK, including retransmissions), all the way up the stack, until they hit the channel - only to be discarded there. This is a waste of resources (CPU and memory).

A solution is to discard the multicast at the transport level. If a message has a flag LOCAL set to false, then TP discards it if

• the message is a multicast message and
• the sender of the message is the local member