Add some missing features of ArmRedmine

* Fix a build error against old libsoup
* Enable to save updated incidents
* Add a function to start ArmRedmine thread

But ArmRedmine isn't enabled yet, no one calls the start() function yet.

When the instance (cache and dbMonitoring) is defined here, its constructor is always executed even if dbMonitoring is not used in this method.
The overhead is slight, but I feel a little sense of discomfort.