Custom rsync tool

[touilleMan]

rsync currently works fine with Parsec mountpoint, however a custom implementation of rsync in Parsec should greatly increase performances:

File change detection in rsync can be typically done by two means:

1. checking file size and modification time (the default mode which is fast)
2. checking file content hash (which is not practical with Parsec given all data must be downloaded back to the computer for hash calculation...)

The idea would be to take advantage of the fact we already have all those information (size, modification time and even per-block hash) on the file manifest.

On top of that, we would also avoid having to go through the mountpoint ;-)

[touilleMan]

implemented in #1175