Introduce boot features sync mode property (true|false)

We changed the behaviour of the boot features to be in sync mode: previously a new thread was created for each boot feature startup.

However, users may complain about this change and the general Karaf startup time.

I will introduce a property (in etc/org.apache.karaf.features.cfg) to let the users decide if they want boot features in sync or async mode (and set sync by default).