External RA improvements

In the upcoming EJBCA 3.9 the External RA is finally getting some long waited improvements.
The CA component will now run as a service in EJBCA. This means that you do most of the configuration in the admin-GUI of EJBCA and that it runs very nicely in a CA cluster. You can also configure multiple external RAs, as many as you need.
Setting up a cluster of external RAs is now very simple, if you have a cluster of two external RAs simply configure two external RA services in EJBCA and you're done. No need to use complicated database clusters etc on the external RAs, each external RA node can be simple and stand-alone.

Installation of the external RA is also much much simpler now. Configure the path to the external RA package in EJBCA and the needed CA service is automatically pulled into EJBCA so it is available to be configured in the Admin-GUI. The only thing that needs some though is the configuration of datasources in your application server.

To summarize:
- Easier to install and configure
- Runs nice in a CA cluster
- Runs nice against multiple external RAs

As an added bonus, it's also now almost trivial for developers to implement new types of external RA messages. Internally it uses java reflection, so all you have to do is implement the message classes and handlers. The rest is handled automatically.