RemoteEx 2.2: Promote platform properties from command to action (#167)

Platform properties are currently a member of the `command` message which is
referred to in the action by digest. This requires the Execution Service to
make a call to the CAS to retrieve the contents of the command if it wishes to
inspect it. The platform properties are commonly used for making routing
decision about the action. Therefore, in order to route an action common
execution service implementations must introduce an additional call to the CAS
to fully hydrate the action and determine where it should be routed.

This commit promotes the platform properties from the command to the action. We
deprecate the platform properties contained within the action and bump the
minor version to version 2.2, following the model for `output_paths`.

Fixes #166

Signed-off-by: Ed Baunton <ebaunton1@bloomberg.net>