Add ExchangeSupervisorActor

**Description** Add ExchangeSupervisorActor without the handling of the failure cases, which will be handled by a new actor which is out of scope for this PR
**Testing** Tests added, tests pass
**Coverage**

- ExchangeSupervisorActor: 100%
- InitializedExchangeSupervisorActor: 93.40%
- DefaultHandshakeActor: 100%
- InitializedHandshake: 100%
Ok. I had a misunderstanding respect this actor. I though `ExchangeSupervisorActor` was managing all the exchanges in course in the system. But I see now its just coordinating the handshake and the micropayment channel. If so, my suggestion is:

* Leave `HandshakeActor` as is.
* Rename `BuyerExchangeActor` to `BuyerMicroPaymentChannelActor`
* Rename `SellerExchangeActor` to `SellerMicroPaymentChannelActor`
* Rename `ExchangeSupervisorActor` to `ExchangeActor`