syncbase API: client watch (VDL only)
syncbase API: client watch (VDL only)

Watch allows a client to watch for updates in the database.

This change contains only the VDL API and implementation stubs.

The Watcher is not embedded directly from the "v.io/v23/services/watch"
package because:
- the store has a more convenient way to get the initial state: grab a
  snapshot, do gets and scans and get the ResumeMarker (as a part of the
  same snapshot) as a point to start
- we want to have a bit different information in a change, for example,
  whether the change came from Sync.
- it is more convenient to use tuples instead of strings for table+row names.

Change-Id: Ib657dc7da2bbf857274d994ffd2d0853b07c587c

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