Change th way the files are sent.

Before that, each file was sent upon modification and xmpp client had to connect each time. We had a lot of connection errors because the file could be modified while it was sent.
Now, we send the file in batch and temporize the list of file to send to avoid problems.

File receiver could sometimes try to write on closed file. We fixed that.

Known bugs/limitations: the sending is not retried when something go wrong and file integrity is still not checked.