upload: 1) session update and filter; 2) add upload description argument

1. (issue #43 (closed)) In order to be able to collect (and provide to developers, who expressly asked for it) stats about each pipeline job's results (and show them directly in the dashboard), we should adjust the upload command so that:

• if filters out packages that have already been uploaded, so only new packages that have actually been added by the pipeline are showed in stats (dashboard)
• it updates the session file in real-time, also in case of upload errors due to Fossology timeouts (which may occasionally happen), so that it can be re-run on the same session in order to upload the missing packages while retaining the previous upload history (otherwise we would have some packages that result as already uploaded and would be excluded from the stats)

2. (issue #44 (closed)) Fossology has a "description" field for every upload, that currently is filled by a4f with a standard hard-coded text ('uploaded by aliens4friends'). It would be very useful to add an option to set a custom description via CLI (eg. one may want to use 'uploaded from pipeline #XXXXXX project OSTC/OHOS/manifest MR #X') in order to be able to identify, in Fossology, which uploads have been uploaded by which pipeline in consequence of which Merge Request.