implement update_overall_statistics() and refactor create_system

[lyuyangh]

part of #433
The goal of this PR is to refactor the DB operations used by create_systems(). The operations are split into three methods:

def db_operations(session: ClientSession) -> None:
      system.save_to_db(session)
      system.save_system_output(system_output_data, session)
      system.update_overall_statistics(metadata, system_output_data, session)

Each method can be run multiple times without causing an issue. They can also be called in any order after the system has been created. This is necessary if we want to treat overall_statistics as cache (previously we were calling it system_info and metric_stats). Following this PR, I will implement a system which calls update_overall_statistics() to update the cache whenever necessary. Note that, update_overall_statistics() doesn't clean up stale resources properly if it is called multiple times. It works for now because it is only called once for each system. This PR is already pretty big so I plan to handle it in the next PR.

[neubig]

LGTM, thanks!