only specify upper key bound when querying load

As a querying node does not always know the lower bound of the queried
segment – determined by the predecessor – let the currently responsible
node provide that bound instead.

affects #71

test/FediChordSpec.hs

@@ -222,12 +222,12 @@ spec = do
                                  ]
                                               }
             qLoadReqPayload = LoadRequestPayload
-                { loadLowerBound = fromInteger 12
-                , loadUpperBound = fromInteger 1025
+                { loadSegmentUpperBound = 1025
                 }
             qLoadResPayload = LoadResponsePayload
                 { loadSum = 3.141
                 , loadRemainingTarget = -1.337
+                , loadSegmentLowerBound = 12
                 }
             requestTemplate = Request {
                 requestID = 2342