fix: Persist data from data.api to Marketplace back-end

Fixed persistence capability in data.api. Any packets sent to data.api with entity.persist set to
a truth value will be immediately persisted to the configured back-end server.  Rather than use the
existing method DataApi.persistNodes(), I created a new method persistNode(). The former will eventually
be used to persist dirty nodes periodically in a batch mode rather than have all nodes designated for
persistence write-through immediately.

refs #95