Add a --rate-config flag pointing to a JSON file for per-client receive
rate limits (bytes/sec and burst bytes). The config is reloaded on SIGHUP,
updating all existing client connections live. The --per-client-rate-limit
and --per-client-rate-burst flags are removed in favor of the config file.
In derpserver, rate limiting uses an atomic.Pointer[xrate.Limiter] per
client: nil when unlimited or mesh (zero overhead), non-nil when
rate-limited.
Document that clientSet.activeClient Store operations require Server.mu.
Updates tailscale/corp#38509
Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>
ca5db865b4