[state machine review] Introduce locks on entities

[ndr-brt]

We need to introduce the possibility to lock an entity when it's waiting for a response from an async operation (e.g. provision for TransferProcess) so that it won't be fetched by the nextForState method.

This will fix this discussion point:

Discussed in #549

^{Originally posted by ndr-brt January 25, 2022}

The state is not moved to an "not to be processed" state before executing an async operation

Problem
This is the problem described here: #538 . When an entity is fetched from the nextForState and an async operation is called, it should be set in a state that's not fetched by the state machine loop, otherwise that can cause useless loops (as described in the issue)
Solution
Define 3 type of states:

• To be processed state: an entity with this state is fetched by the nextForState, processed and moved to a processing state (async operation), to another to be processed state (sync operation) or to a final state (nothing else should be done)
• Processing state: an entity with this state is waiting for an asynchronous response, it's not fetched by the nextForState.
• Final state: an entity with this state has reached the terminus and there should stay, e.g. states like COMPLETED, ENDED, ERROR, ...

[ndr-brt]

Superseded by #615 as stated in #690