Decompose `faabric::Message`

[Shillaker]

The faabric::Message protobuf object is bloated now and has become a dumping ground for lots of context. Having a single message format is very convenient, and necessary because of the breadth of different types of calls we support.

I think we can refactor as follows:

• Move all Faasm-specific flags into custom serialised contexts held in the BatchRequest::contextData field. This is what we do with OpenMP, and can be applied to all SGX- and Python-related fields on the Message.
• Move all MPI-related fields into a nested MPI context.
• Collect all feature flags into a nested flags object (isAsync, isStatusRequest etc.).
• Move all batch-wide properties onto the BatchRequest, e.g. snapshotKey, appId, masterHost, executesLocally, groupId
• Create a separate protobuf object for exec-graph data.