Refactor Save API to replace "scalar:metadata:<predicate>" with "scalar:<predicate>"

Bring in line with the RDF API output--"scalar:metadata:<predicate>"
was only used by the Save API.  Note that both
"scalar:metadata:<predicate>" and "scalar:<predicate>" are presently
supported, to allow for graceful migration. TODO: remove the
"scalar:metadata:<predicate>" protocol once migrations are complete