persist ConceptClusters for when backend shuts down

[fmatthies]

There is a HashMap<String, Thread> conceptClusterProcesses in ConceptClusterService that will be populated with running/finished processes of createSpecificGraphsInNeo4j. This map is used to check for said processes from ConceptClusterApiDelegateImpl (and therefore the frontend).

However, when the backend terminates for any reason this map won't be restored at the moment and the information is lost. But fixing this shouldn't be a problem since the clusters are stored in Neo4j and we could get the information about what clusters are present from there.