wip split Message in two

give context

TODO spec points

- OutgoingMessage’s `id` and `timestamp` properties are now optional
- OutgoingMessage does not have `fromEncoded*` methods

- IncomingMessage’s `connnectionId` property is now non-optional
- IncomingMessage does not have constructors

the top stuff are versions that don't remove any properties, per

textile/features.textile:** @(RSL1j)@ When @message@ objects are provided, any valid @message@ attribute (that is, an attribute specified in "TM2":#TM2) that is supplied by the caller must be included in the encoded message. (This does not mean it must be included _unaltered_; for example the @DaTa@ and @encoding@ will be subject to processing per "RSL4":#RSL4)

but want to understand that better to see if it's really needed, because there
probably are properties that we could remove

be clear that the IDL describes the public API of the library

TODO what about re-publishing?

TODO rename Message to MESSAGE where appropriate

TODO mention that ProtocolMessage shouldn’t even be in the spec and that we should just link to the ProtocolMessage definition in the protocol

The protocol documentation needs to be up to date