ipn/{ipnlocal,localapi}: ensure watcher is installed before /watch-ipn-bus/ responds with 200

This change delays the first flush in the /watch-ipn-bus/ handler
until after the watcher has been successfully installed on the IPN
bus. It does this by adding a new onWatchAdded callback to
LocalBackend.WatchNotifications().

Without this, the endpoint returns a 200 almost immediatly, and
only then installs a watcher for IPN events.  This means there's a
small window where events could be missed by clients after calling
WatchIPNBus().

Fixes tailscale/corp#8594.

Signed-off-by: salman <salman@tailscale.com>
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