Add route permissions

The `client.perms` struct is left unchanged. We simply map Import
and Export semantics to existing Publish and Subscribe ones.

Signed-off-by: Ivan Kozlovic <ivan@synadia.com>

test/configs/srv_a_perms.conf

@@ -0,0 +1,25 @@
+# Cluster Server A
+
+listen: 127.0.0.1:5222
+
+cluster {
+  listen: 127.0.0.1:5244
+
+  authorization {
+    user: ruser
+    password: top_secret
+    timeout: 0.5
+    permissions {
+      import: "foo"
+      export: ["bar", "baz"]
+    }
+  }
+
+  # Routes are actively solicited and connected to from this server.
+  # Other servers can connect to us if they supply the correct credentials
+  # in their routes definitions from above.
+
+  routes = [
+    nats-route://ruser:top_secret@127.0.0.1:5246
+  ]
+}