InMemorySubscriber should specify what version of the subscription protobuf it is working with

[tamarafischer]

Currently we have version 2 and version 3 of the subscription protobuf.
version 2 is the one actually used, as of typing this issue. I am not sure who is using version 3.

InMemorySubscriber should specify the version or maybe have 2 versions of Subscriber, otherwise we might have problems understanding the payload.

[stevenhartley]

InMemorySubscriber is based off UTransport that is based off UMessages that are a part of 1.6.0 of uProtocol that will only be using in deployments that have v3 or later of uSubscription as v2 is not using Eclipse-uProtocol but the older closed source version of uProtocol.
That being said, we do need to consider supporting updated versions of the proto in the future (when we go from v3 to v4 whenever that might happen).

[stevenhartley]

This is addressed by #148 and the PR #158 .