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.

{
    if (fChangeCached)
        return nChangeCached;
    nChangeCached = pwallet->GetChange(*this);
    fChangeCached = true;
    return nChangeCached;
}
bool CWalletTx::InMempool() const
{
    LOCK(mempool.cs);
    if (mempool.exists(GetHash())) {
        return true;
    }
    return false;
}
bool CWalletTx::IsTrusted() const