syncbase/vsync: bootstrap sync metadata for SyncGroup
syncbase/vsync: bootstrap sync metadata for SyncGroup

Add the bootstrapping of sync metadata when a SyncGroup is created or
joined.  It notifies sync, through the watcher, of pre-existing database
entries that match the SyncGroup prefixes.  Such entries must be added
to the sync metadata (DAG & logs) at their current versions.

Change-Id: I37b4277a370fb31c520bbb858664e0ad078cce74

vanadium/go.v23,vanadium/go.v23,vanadium/go.v23