[[#133]]: atomic read/writes rework to avoid deadlocks in Policies.

Changed

Aggregate:
- apply_events has been removed, and its default implementation moved to a method of AggregateState.
AggregateManager:
- lock_and_load acquires a lock, then loads into memory the AggregateState
  preventing other atomic accesses. Dropping the AggregateState releases the resource.
- lock has been removed in favour of lock_and_load.
- handle_command does not return the AggregateState anymore. This avoids race conditions where the
  returned state is already outdated, if another concurrent access has taken place at the same time.
- apply_events has been removed, and its default implementation moved to a method of AggregateState.
- load has been changed to return a Result<Option<_>, _>, to explicit expose errors.
AggregateState:
- new lock field, which contains the exclusive access guard created when lock_and_loading.
  All other fields are now private for better encapsulation.
- new takes no argument, and generates a new state with a random UUID.
- with_id generates a new state with the given UUID.
- apply_store_events applies the given list of Events on self.
- set_lock and take_lock methods to set and get the lock,
  to use when overriding the AggregateManager functionality.

Commits