Fix threads not totally released

Issue

When a thread has finished its work with a gien document to sync, it is not really freed.
As a result, syncing a lot of files will fail on:

OSError: [Errno 24] Trop de fichiers ouverts: '...'
OSError: [Errno 24] Too many opened files: '...'

Monitoring

To monitor such behavior, I used this command on GNU/Linux:

for i in $(seq 4096); do ps a | grep '[n]xdrive' | cut -d ' ' -f 1 | xargs lsof -p 2>/dev/null | wc -l && sleep 1; done

The number of descriptors is bigger and bigger, until the limit is hit and Drive hard crash.