MM-8607: Add ability to turn off non-critical services when under load; cluster support (#13267)

* MM-8607: Add cluster support for server busy status

MM-8607: Busy.Cluster should be non-exported

MM-8607: rename logging field seconds -> expires_sec

MM-8607: each node clears its own busy flag

MM-8607: ensure unit tests are not sensitive to test machine speed

* MM-8607: chg comment to force CI rebuild

model/system.go

@@ -51,6 +51,7 @@ type SystemECDSAKey struct {
 	D     *big.Int `json:"d,omitempty"`
 }
 
+// ServerBusyState provides serialization for app.Busy.
 type ServerBusyState struct {
 	Busy       bool   `json:"busy"`
 	Expires    int64  `json:"expires"`