When handling updates to existing entries, it is often necessary to know
what the previous value was. NetworkDB knows the previous and new values
when it broadcasts an update event for an entry. Include both values in
the update event so the watchers do not have to do their own parallel
bookkeeping.
Unify the event types under WatchEvent as representing the operation kind
in the type system has been inconvenient, not useful. The operation is
now implied by the nilness of the Value and Prev event fields.
Signed-off-by: Cory Snider <csnider@mirantis.com>
