derp/derpserver: support global rate limiting independent of per-client

This commit enables the operator to set a global rate limit without any
per-client.

Updates tailscale/corp#40962

Signed-off-by: Jordan Whited <jordan@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f