TransferProcess: Do we need the REQUESTED_ACK state?

[ndr-brt]

Going through the TransferProcess state machine I noticed this REQUESTED_ACK state.
This state, according to the code and the docs, can be reached only on the consumer side, and at the moment there's only one point where there's such state transition:
after the provision is complete, the TP is set in REQUESTED state and the data request is sent to the provider, if the response is successful, the TP is moved to REQUESTED_ACK and moved suddenly to IN PROGRESS (or STREAMING).

There's also a checkProvisioned() method that fetches TPs in such state to move them in IN PROGRESS (or STREAMING), but probably never won't find anything.

To follow the "state machine pattern" that was defined in #613 probably we would remove the REQUESTED_ACK state, and for the PROVISIONED TP the process should be:

• put it in REQUESTED state
• send data request to provider
• if response is successful, move it to IN_PROGRESS or STREAMING
• if response is failed, move it to PROVISIONED if there's a retry strategy set or to ERROR elsewhere.