Remove GenServer clone on message reception

[ElFantasma]

GenServers were cloned before calling handle_cast and handle_call to support a "rollback" functionality if the callback failed on error.

As a result:

• It was causing a major performance issue on GenServers with a large state and with heavy message traffic.
• It required all GenServers to implement Clone.
• This feature showed it was not of use; eventually it is preferred to end the GenServer on errors, instead of rolling back.
• And actually the feature was not reliable, as the internal state of the GenServer could consist of pointers to external structures (eg. an Arc<Mutex<Store>>) that would suffer modifications during the callback execution and won't rollback on error.

So this removes the cloning of the state on each message, also removes the requirement to implement Clone on every GenServer, and also, it borrows self for message callbacks instead of transferring ownership, making the use a bit more flexible (no need to return a "new_state" on each callback call).

This last change breaks backwards compatibility. Code using this library will need to modify a bit the implemented GenServers