Extract FedoraService from FedoraObjects and FedoraDatastreams

The tests on this branch are not passing, because I'm unsure how to get spring to actually inject this bean: https://github.com/jcoyne/fcrepo4/blob/service/fcrepo-legacy-api/src/main/java/org/fcrepo/api/legacy/service/FedoraService.java#L59

Can someone who knows more about Spring take a look at this?