Add explicit format to magix spec

[altavir]

Based on recent discussions, I suggest making a final change to magix package format:

• Add mandatory format string field. Which will replace the origin field. The format does not point to a specific endpoint, it only specified the payload format. Plugins can evaluate messages based on this field.

• Replace mandatory origin by optional originID (or sourceID). Which specified the unique ID of a specific endpoint (either system or service).

• Deprecate target field.

The reasoning behind those changes is the following:

• It is possible to have several connected systems of the same type. For example, there could be two TANGO connectors. In current spec, it is not possible to distinguish between them without creating complex parsing rules for origin.

• We need to be able to identify systems. For example, we will need to be able to subscribe to specific endpoint events. Also we will want to have services that will be able to resolve direct connection from API (like CORBA does in TANGO and LDAP in DOOCS). This direct connection is needed for binary transfer.

• Currently there are no cases where the endpoint explicitly creates a message for another endpoint. We are working strictly on subscription policy. Therefore target is meaningless.

[altavir]

Even better to use sourceEndpoint for origin in order to make terminology clear.

[Ingvord]

sourceEndpoint sounds OK for me

target is very useful exactly in cases when specific target is being addressed, and in fact is already used in AXSIS-XES for controlling motors. We can replace it with targetEndpoint