LMS-1966 Refactoring DSS to make alternate top-level data set handlers possible.

...sx/cisd/etlserver/AbstractTopLevelDataSetRegistrator.java

...ver/source/java/ch/systemsx/cisd/etlserver/ETLDaemon.java

...va/ch/systemsx/cisd/etlserver/ETLServerPluginFactory.java

.../systemsx/cisd/etlserver/ITopLevelDataSetRegistrator.java

...rce/java/ch/systemsx/cisd/etlserver/ThreadParameters.java

...cisd/etlserver/TopLevelDataSetRegistratorGlobalState.java

...ch/systemsx/cisd/etlserver/TransferredDataSetHandler.java

...emsx/cisd/etlserver/api/v1/DataSetTypeToPluginMapper.java

.../ch/systemsx/cisd/etlserver/api/v1/PutDataSetService.java

...ystemsx/cisd/etlserver/TransferredDataSetHandlerTest.java