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: 4/5
Change-Id: Ib316fd61c0c45211836f532ac4677507002c5f05
3 files changed