minor refactoring: simplify session access in *Server classes

.../systemsx/cisd/openbis/generic/server/AbstractServer.java

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

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

...a/ch/systemsx/cisd/openbis/generic/server/ETLService.java

...is/generic/server/authorization/AuthorizationAdvisor.java

.../cisd/openbis/generic/server/dataaccess/db/FilterDAO.java

...penbis/generic/shared/authorization/ISessionProvider.java

.../systemsx/cisd/openbis/plugin/demo/server/DemoServer.java

...msx/cisd/openbis/plugin/generic/server/GenericServer.java

.../openbis/plugin/phosphonetx/server/PhosphoNetXServer.java