disable self connections by default

now that the event manager can create files, self connections may create
even more issues than before

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

sftpgo.json

@@ -18,6 +18,7 @@
     "max_total_connections": 0,
     "max_per_host_connections": 20,
     "whitelist_file": "",
+    "allow_self_connections": 0,
     "defender": {
       "enabled": false,
       "driver": "memory",