Use more processors by default (5 -> 10)

Here, a processor is a thread than handle one item to sync. By default, 5 are used:

• local file
• local folder
• remote file
• remote folder
• another generic one to handle more data

We could use a higher number, let's say 10. It will unlock 5 new generic processors.

This will also fix that issue: #541 (sync is slow).