CORDA-1564 Clean up old sessions when retrying flows. (#3278)

node/src/main/kotlin/net/corda/node/services/statemachine/SingleThreadedStateMachineManager.kt

@@ -354,6 +354,10 @@ class SingleThreadedStateMachineManager(
             null
         }
         externalEventMutex.withLock {
+            // Remove any sessions the old flow has.
+            for (sessionId in getFlowSessionIds(currentState.checkpoint)) {
+                sessionToFlow.remove(sessionId)
+            }
             if (flow != null) addAndStartFlow(flowId, flow)
             // Deliver all the external events from the old flow instance.
             val unprocessedExternalEvents = mutableListOf<ExternalEvent>()