all: use immutable node view in read path

This commit changes most of our (*)types.Node to
types.NodeView, which is a readonly version of the
underlying node ensuring that there is no mutations
happening in the read path.

Based on the migration, there didnt seem to be any, but the
idea here is to prevent it in the future and simplify other
new implementations.

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

hscontrol/policy/v2/policy_test.go

@@ -47,7 +47,7 @@ func TestPolicyManager(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			pm, err := NewPolicyManager([]byte(tt.pol), users, tt.nodes)
+			pm, err := NewPolicyManager([]byte(tt.pol), users, tt.nodes.ViewSlice())
 			require.NoError(t, err)
 
 			filter, matchers := pm.Filter()