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

config/config.go

@@ -73,6 +73,7 @@ func init() {
 			ManageUsers:      1,
 			SSLMode:          0,
 			TrackQuota:       1,
+			PoolSize:         0,
 		},
 		HTTPDConfig: api.HTTPDConf{
 			BindPort:    8080,