Improved memory usage and readability

Extract the parts for loading the file from application server /
presentation server into two new private methods.
Not only is the code better readable (currently, the very important LOAD
call at the end of the method almost vanishes after all that file
loading code)  - it also saves memory. Currently, when the LOAD message
is processed, not only the EXCEL_DATA xstring, but also BIN_TAB, the
auxiliary table of bytes which is retrieved from GUI_UPLOAD, is kept in
the memory as a redundant copy of the raw data, although during the
parse process it is not needed any more.