ipn/ipnlocal: signal nodeBackend readiness and shutdown

We update LocalBackend to shut down the current nodeBackend
when switching to a different node, and to mark the new node's
nodeBackend as ready when the switch completes.

Updates tailscale/corp#28014
Updates tailscale/corp#29543
Updates #12614

Signed-off-by: Nick Khyl <nickk@tailscale.com>
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