syncbase API: client watch, addressing v.io/c/13615 comments
syncbase API: client watch, addressing v.io/c/13615 comments

Discussed offline, decided to use the general watch API.

Change-Id: I6ac70ab11f3703188ab15b73e9ef7867a9605d0c

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