Update position on bulk loading

There was some discussion on TinkerPop's position on bulk loading on the dev list:

https://lists.apache.org/thread.html/1f91d87d2af942497e2c0ca4c3d535752b21a45a945e7750e912e982@%3Cdev.tinkerpop.apache.org%3E

Basically, we will rely on graph providers to supply their own bulk loaders and will not promote generic TinkerPop methods for doing so. We will keep BulkDumperVertexProgram so that providers can rely on that bulk loading infrastructure if they like. BulkLoaderVertexProgram can be deprecated.