Introduce concept of read-only variables

[Scottapotamas]

Introduce concept of read-only variables

[Scottapotamas]

946d8de adds support for read only variables.

Remaining work is to modify the handshake process to provide this information to the UI.

[Scottapotamas]

Refactored the handshake callbacks as part of the (off-branch) 2ad87e1 commit. This code has not been tested on hardware, and doesn't have written unit tests, but does compile without warnings.

Breaking changes:

• The previous message ID's used ("dm" and "dv") have been replaced by "dmr"/"dmw" and "dvr"/"dvw" for the read-only and the read/writeable ones.
• It's up to the UI to request each set and handle the responses accordingly.
• The previous "dms" for indicating start of the stream (do we actually need that), is now "dmrs"/"dmws".
• The previous packets containing the list of messageID's "dml", is now namespaced "dmrl"/"dmwl".

Remaining work:

• Test on hardware,
• Test against the UI once support is available in the connections manager,
• Write unit tests for each of the callbacks and expected behaviour once stabilised,
• Merge a working implementation into master.

[Scottapotamas]

This was merged in #62, and #47 somewhat encompases the higher level test requirements.

UI support should be tracked outside the scope of this repo.