Support constant peer IDs upon restarting proxy

[t-lin]

• The key flags are backwards-compatible with the pre-existing positional arguments, but they must be specified before them.
• This feature is desirable in cases where a proxy, even if its in a container, restarts (e.g. the host the container runs on reboots). In such a case, the proxy should ideally retain the same ID. This prevents monitoring systems from thinking service churn has occurred, and more accurate represents the flaky nature of the host.
• Resolves #21 for the proxy service