we now have VertexProgram.workerStartup() and VertexProgram.workerShutdown(). Implemented for both TinkerGraphComputer and GiraphGraphComputer. All tests pass. TraversalVertexProgram makes use of it to create a Traversal cache that is local to the worker -- no more thread local. More work to be done on updating memory in these methods --- we may just make it so that these are read-only structures (much like the final resultant memory) of ComputerResult. Lots of other random tweaks.  Fixed #318.