Don't get stuck at "Syncing 0%" when adding a new folder

The number of copiers and pullers is set to default at config loading
time, but the new folder configuration doesn't pass through config
loading so we start up with 0 copiers and 0 pullers and hence get stuck.
I moved the default handling to the puller itself instead. I think this
way is also cleaner as we get to keep the 0 in the config and the puller
gets to decide the defaults on it's own.

lib/config/config_test.go

@@ -89,8 +89,8 @@ func TestDeviceConfig(t *testing.T) {
 				Devices:         []FolderDeviceConfiguration{{DeviceID: device1}, {DeviceID: device4}},
 				ReadOnly:        true,
 				RescanIntervalS: 600,
-				Copiers:         1,
-				Pullers:         16,
+				Copiers:         0,
+				Pullers:         0,
 				Hashers:         0,
 				AutoNormalize:   true,
 				MinDiskFreePct:  1,