TUN-5128: Enforce maximum grace period

This maximum grace period will be honored by Cloudflare edge such that
either side will close the connection after unregistration at most
by this time (3min as of this commit):
 - If the connection is unused, it is already closed as soon as possible.
 - If the connection is still used, it is closed on the cloudflared configured grace-period.

Even if cloudflared does not close the connection by the grace-period time,
the edge will do so.

connection/connection.go

@@ -18,6 +18,7 @@ import (
 const (
 	lbProbeUserAgentPrefix = "Mozilla/5.0 (compatible; Cloudflare-Traffic-Manager/1.0; +https://www.cloudflare.com/traffic-manager/;"
 	LogFieldConnIndex      = "connIndex"
+	MaxGracePeriod         = time.Minute * 3
 )
 
 var switchingProtocolText = fmt.Sprintf("%d %s", http.StatusSwitchingProtocols, http.StatusText(http.StatusSwitchingProtocols))