add one minute chart + refactor rpc

- add one minute charts
- update disk io to use bytes
- update hub and agent connection interfaces / handlers to be more
flexible
- change agent cache to use cache time instead of session id
- refactor collection of metrics which require deltas to track
separately per cache time

agent/client_test.go

@@ -301,7 +301,7 @@ func TestWebSocketClient_HandleHubRequest(t *testing.T) {
 				Data:   cbor.RawMessage{},
 			}
 
-			err := client.handleHubRequest(hubRequest)
+			err := client.handleHubRequest(hubRequest, nil)
 
 			if tc.expectError {
 				assert.Error(t, err)