Mavenize the java aspect a bit more.

First, move the java source to what maven's compiler expects.  Then
use it to compiled both main and test code, & run the unit tests.

It also makes a 'fat' jar for us, which we explode to make ./classes.
This avoids the need for jars/runtime_classes.

Askari::Packaging.java_classes_dir is a Pathname now.

Several maven plugin versions were updated a la
`mvn versions:display-plugin-updates`

Ditch ./jars, create test classpath in regular maven build and push
loading it into spec_helpers/test_classpath.rb.