Aggregator meta data in properties

The MessageMulticaster creates an aggregation id and stores this value in message properties. Normally this does not prove to be an issue but when using invm transport it may cause issues.

When the message is sent to multiple services, using pass by reference semantics, then all services will see the same properties and may, depending on timing, see the same aggregation id.

The aggregation value should really be in the message context as this section is always duplicated when creating a reference, never shared. The problem we face is that users may already be referencing the property, especially if they are creating a fresh response message, so we need to think about handling this.