dataprovider: remove transaction for quota update

The update is atomic so no transaction is needed.
Addionally a transaction will ask for a new connection to the pool
and this can deadlock if the pool has a max connection limit too low.

Also make configurable the pool size instead of hard code to the cpu number.

Fixes #47

sftpgo.json

@@ -30,7 +30,8 @@
     "connection_string": "",
     "users_table": "users",
     "manage_users": 1,
-    "track_quota": 2
+    "track_quota": 2,
+    "pool_size": 0
   },
   "httpd": {
     "bind_port": 8080,