Move node_id generation into State (#2401)

[panh99]

Description

This PR does two things: 1) generate node_id and workload_id as a random 64-bit unsigned integer instead of a positive int64; 2) move the generation of node_id code into state.
However, it may not be super necessary to use "real" uint64, given that sqlite3 does not natively support uint64. We can always use 63 bits instead of all 64 bits if the information loss due to 1 bit less is trivial.

Refactoring State.register_node(self, node_id: int) -> None and State.unregister_node(self, node_id: int) -> None to State.create_node(self) -> int and State.delete_node(self, node_id: int) -> None. The two methods are renamed to fit their new functionalities.

Move the node_id generation within the State.create_node().

Make node_id true uint64, via random.getrandbits(64).

Update InMemoryState and SqliteState.

Store uint64 values as int64 in SqliteState and convert them back to uint64 on retrieval.

Update corresponding tests, comments, names, and protocols.

[danieljanes]

Lgtm!