Implement load/save function for upload stats

[defnax]

Half finished libretroshare PR is here -> #210

Gui PR is here:
RetroShare/RetroShare#3089

we need save & load function for the stats:

• add to p3FileDatabase class the functions to update the upload stats, as called from ftServer (no mutex needed in ftServer for that since the one is p3FileDatabase will be used)
• create a new RsItem in rsfilelistitems.h to store the upload stats.
• add the proper lines to load/save these stats in p3FileDatabase::loadList() and saveList() using these created items

Update from 21.12.2025
New PR -> #220

[defnax]

PR #220 is merged and issue is solved