Cleanup message handling code

[Maelkum]

This code makes a number of changes to the message types and message handling code.

• messages no longer have an explicit Type field that can be set - having this and having to set it manually is prone to error (someone might use a certain model and inadvertently set the wrong message type/ID). Now this is done automagically on JSON marshalling
• messages no longer have an explicit From field telling us who sent the message - this is handled by the node
• invalid/unused message types are removed
• we use generics (!) to simplify message decoding, and
• message processing functions now receive actual types they need and not an obscure byte slice they need to manually unmarshal
• some verbosity added by defining custom marshallers but wanted to keep backwards compatibility for the moment and have the message data and message type field as top-level fields; worth it for less clutter in node package IMO
• added differentiation between published messages and those we receive via direct communication; added some validation there - we don't allow just any message to be published as it could lead to issues (e.g. random worker publishes MessageExecute or MessageDisbandCluster)
• fix handling of workspace/peer db/function db flags - config/default values are now handled correctly