When importing a watch-only address over `importmulti` with a specific timestamp, the wallet's `nTimeFirstKey` will be set to 1.
I think we should add a optional timestamp to `CWallet::AddWatchOnly()` and use it when importing addresses or pubkeys.

{
        SetTx(MakeTransactionRef());
        Init();
    }
    CMerkleTx(CTransactionRef arg)
    {
        SetTx(std::move(arg));
        Init();
    }
    /** Helper conversion operator to allow passing CMerkleTx where CTransaction is expected.
     *  TODO: adapt callers and remove this operator. */
    operator const CTransaction&() const { return *tx; }
    void Init()
    {
        hashBlock = uint256();
        nIndex = -1;
    }
    void SetTx(CTransactionRef arg)
    {
        tx = std::move(arg);
    }
    ADD_SERIALIZE_METHODS;
    template <typename Stream, typename Operation>
    inline void SerializationOp(Stream& s, Operation ser_action) {