add configure(conf) to pregelix API, called once per iteration

@anbangx @Elmira88 @JavierJia @Nan-Zhang 
This commit was just pushed to @sigmod as a change in the pregelix API.  It will allow us to replace all our initVertex() stuff with just the `configure(conf)` function, which will be called once per iteration (not once per iteration per vertex like compute() and open() are).

Let's clean up the initVertex() code all over the place and push the updates to this branch.