awe-client check and download prerequisite data (e.g. refdb)

1. refdb url is defined in workunit sruct
2. dbs will be downloaded to conf.DATA_PATH
3. symbolic link will be made in the work dir of that workunit for each prerequisite file
(note: REFDBPATH is no need to set on awe client, otherwise, it should be set to conf.DATA_PATH; make sure application code can find refdbs at CWD or conf.DATA_PATH).