syncbase/watch: Return initial state for empty ResumeMarker.
syncbase/watch: Return initial state for empty ResumeMarker.

Empty ResumeMarker is a convenience for Scan followed by Watch,
returning the initial state of the watched row set as the first
change batch, as specified in v.io/v23/watch.

Fixes v.io/i/693 and most of v.io/i/917

MultiPart: 1/5
Change-Id: Ibfb7a24d31d9d60878bba64c7f20caa45531e8c7

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref