DS-2066 : Maven plugin updates and general POM cleanup

This updates all Maven plugin versions (to latest versions I could find on Maven Central) in all of our POM files. In most cases, this is just a simple version change...but a few plugins have been renamed or work slightly differently, including:

* maven-license-plugin is now called "license-maven-plugin"
* gmaven-plugin is now called "groovy-maven-plugin", and its config requires some minor changes

I've also performed some general POM cleanup, including:
* Improving readability / alignment (by replacing tabs with spaces and similar)
* Renamed a few POMs (OAI, REST) whose names were slightly non-standard (based how others are named)

Finally, I've updated our Sonatype 'oss-parent' to version 9 (we were on version 7).

I've tested all these changes using both the default build command:
        
        mvn clean package

and the build command to check license headers & perform all unit tests:

        mvn clean package license:check -Dmaven.test.skip=false

Everything builds/tests successfully on Linux.

This does need a quick sanity review by others, and perhaps some quick build tests on both Windows and Mac OSX, just to ensure there's no OS-specific build issues encountered by all these plugin updates.

If possible, I'm hoping we can get this merged sooner rather than later, since it touches a large number of POMs, and is therefore likely to encounter conflicts as we get closer to the 5.0 release deadlines, etc.