feat(PocketIC): PocketIC can recover from a state directory after being killed

[mraszyk]

This PR makes PocketIC able to recover from a state directory after being killed:

• the time is loaded from the latest state (checkpoint) instead of PocketIC metadata (which might not be updated due to a crash);
• PocketIC metadata (topology) is eagerly stored on disk after every topology change (subnet creation);
• the argument of the endpoint /instances/ takes an additional optional field allow_incomplete_state specifying if incomplete state (e.g., resulting from not deleting a PocketIC instance gracefully) is allowed.

Warning! Recent state changes might be lost if a PocketIC instance is not stopped properly.