(core) Remove some usused files.
(core) Trying out actor-based mechanism for device comms.
(actors) generalise the DeviceActor to a directory service

for looking up other actors.
(actors) Add new Forward message type to directory.

Allows users of the directory to use the directory without having to
manage the previously looked-up ActorRefs, or using an ask pattern.
(core) Provide a Response trait, paralleling Request
(actors) A Gateway Actor Directory
(modbus) Modbus implementations of Device and Gateway actors.
(actors) Props for the directory actors
(actors) ensure sender receives a reply
(actors) The actor for managing a persistent request.
(actors) little notes to self
(refactor) Big changes - work in progress
(refactor) Re-implement the missing request actor manager.

Work in progress
(refactor) Change DeviceBus to ResponseBus
(capture) RequestActor tests
(capture) RequestActorManager tests
(tests) Use standard fake response bus
(capture) move test file location
(tests) remove unecessary test
(scheduling) Reduce test size.

As it was quite slow