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.

{
        CWalletTx& wtx = *(item.second);
        LOCK(mempool.cs);
        CValidationState state;
        wtx.AcceptToMemoryPool(maxTxFee, state);
    }
}
bool CWalletTx::RelayWalletTransaction(CConnman* connman)
{
    assert(pwallet->GetBroadcastTransactions());
    if (!IsCoinBase() && !isAbandoned() && GetDepthInMainChain() == 0)
    {
        CValidationState state;
        
        if (InMempool() || AcceptToMemoryPool(maxTxFee, state)) {
            LogPrintf("Relaying wtx %s\n", GetHash().ToString());
            if (connman) {
                CInv inv(MSG_TX, GetHash());
                connman->ForEachNode([&inv](CNode* pnode)
                {
                    pnode->PushInventory(inv);
                });
                return true;
            }
        }
    }
    return false;
}
set<uint256> CWalletTx::GetConflicts() const