Prepare for swmr and mpiio

Created by: dominikwerder

As preparation for SWMR and parallel writing, the HDF file and the HDF objects are created, then the file is closed and re-opened as required by the HDF API. The classes CommandHandler and HDFFile therefore had to change. A new helper Msg is added in order to unify message handling and to allow better performance testing. New types, which will only be used in SWMR and parallel, include HDFIDStore and CollectiveQueue and are added here already because they appear in the public interface of HDFWriterModule.