[LMS-1507] improved speed of loading datasets by codes;

...msx/cisd/openbis/generic/server/AbstractCommonServer.java

...ch/systemsx/cisd/openbis/generic/server/CommonServer.java

...openbis/generic/server/business/bo/ExternalDataTable.java

...penbis/generic/server/business/bo/IExternalDataTable.java

...d/openbis/generic/server/dataaccess/IExternalDataDAO.java

...x/cisd/openbis/generic/server/dataaccess/IGenericDAO.java

...eneric/server/dataaccess/db/AbstractGenericEntityDAO.java

...openbis/generic/server/dataaccess/db/ExternalDataDAO.java

...enbis/generic/server/dataaccess/db/VocabularyTermDAO.java

...a/ch/systemsx/cisd/openbis/generic/shared/dto/DataPE.java

...temsx/cisd/openbis/generic/shared/dto/ExternalDataPE.java

...ystemsx/cisd/openbis/generic/server/CommonServerTest.java

...bis/generic/server/business/bo/ExternalDataTableTest.java