Allow remote clients to hint for optimal node when running Server Task

As suggested by Radim on the Hibernate mailing list, for OGM:

• http://lists.jboss.org/pipermail/hibernate-dev/2018-February/017177.html

See also follow ups. We'll likely want to be able to have the task definition on the Hot Rod client hint that it's going to operate on one specific key (simple case) or a group of keys (more complex but still beneficial).