Add a client method to discard all object data

[rafa-be]

Objects' lifetimes match the client's lifetime.

However, in some use case, it would be nice to be able to discard all previously objects when those are no longer needed.

fut = client.submit(function, args)
fut.result()

client.clear_objects()

fut.result()  # raises an exception, the future is no longer valid

fut = client.submit(function_2, args_2)
fut.result()

This would particularly speedup Parfun, as the library is creating more clients than required to avoid memory leaks.