[FMB] Simplify the sync process both on force sync watcher & the shared folder advertiser

+ Split the folder watch in 2 parallel lanes: keep for updates & actually propagate updates further.
+ The force sync only acts when the syncing that's not progressing it's the known one.

More in detail the last case:
+ t'0: changes A in shared folder. Status is syncing
+ t'5: communicate the update.
+ t'10: force sync kicks in and starts paying attention to sync status
+ t'28: update A gets ACKd.
+ t'29: update B. Status is syncing.
+ t'40: force update tells the controller to force sync after only 10 seconds because of update B since it believes it's because of update A.