Refactor MapFunctions enum to be more general

In preparation for refactors like GIRAPH-469, lets make some of the boilerplate code in and around GraphMapper more general and behavior-oriented and less Hadoopy.

To this end, lets make the MapFunctions enum into something more general we can make use of in multiple driver implementations including those that are not based on MRv1 or GraphMapper.

This passes 'mvn verify' on today's trunk but the giraph-accumulo part of the build fails. After grepping the accumulo code, I don't see anything affected by MapFunctions so I'm assuming this is an unrelated matter.