veyron/services/store: Authorize store clients.
veyron/services/store: Authorize store clients.

Specifics:
 - the store server can be configured with one of the standard
   authorizers.
 - no change to the labels on store or watch methods; they remain
   accessible only to clients with admin privileges.
 - change the store dispatcher to the standard SoloDispatcher.

Change-Id: Ib853c40dbd26bb7aaaacf45a85c2dd66fbbc5ca6

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