ServiceHelper: report new services only in debug

The plethora of INFO messages every time a new context is created are
rather overwhelming, especially during unit testing when a new context
is spun up for each and every test.

This change will greatly decrease the amount of traffic on the console
under normal circumstances. The service creation messages are still
easily available by first setting:

  System.setProperty("scijava.log.level:org.scijava.service", "debug");

With such a property set, the ServiceHelper will emit messages at debug
level without infecting the entire JVM with debug level messages.