lntest+lntest: inherit node's public states when restarting it

This commit changes how a node's state is created during startup. When
it already has states, only its internal states are reset since these
info are only useful for the finished test. The name `InitRPCClients` is
changed to `Initialize` to reflect the fact that it's more than
initializing RPC clients.

lntest/itest/lnd_rpc_middleware_interceptor_test.go

@@ -560,7 +560,7 @@ func middlewareMandatoryTest(ht *lntemp.HarnessTest, node *node.HarnessNode) {
 	// test case. So we need to do the wait and client setup manually here.
 	conn, err := node.ConnectRPC()
 	require.NoError(ht, err)
-	node.InitRPCClients(conn)
+	node.Initialize(conn)
 	err = node.WaitUntilServerActive()
 	require.NoError(ht, err)