[#905] morph/client: Fetch NNS hash once on init

NNS contract hash is taken from the contract with ID=1.
Because morph client is expected to work with the same chain,
and because contract hash doesn't change on update, there is no need to
fetch it from each new endpoint.

Change-Id: Ic6dc18283789da076d6a0b3701139b97037714cc
Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

pkg/morph/client/constructor.go

@@ -145,6 +145,11 @@ func New(ctx context.Context, key *keys.PrivateKey, opts ...Option) (*Client, er
 	if cli.client == nil {
 		return nil, ErrNoHealthyEndpoint
 	}
+	cs, err := cli.client.GetContractStateByID(nnsContractID)
+	if err != nil {
+		return nil, fmt.Errorf("resolve nns hash: %w", err)
+	}
+	cli.nnsHash = cs.Hash
 	cli.setActor(act)
 
 	go cli.closeWaiter(ctx)