appc: add flag shouldStoreRoutes and controlknob for it

When an app connector is reconfigured and domains to route are removed,
we would like to no longer advertise routes that were discovered for
those domains. In order to do this we plan to store which routes were
discovered for which domains.

Add a controlknob so that we can enable/disable the new behavior.

Updates #11008
Signed-off-by: Fran Bull <fran@tailscale.com>
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