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.

{
    AssertLockHeld(cs_wallet); 
    setLockedCoins.erase(output);
}
void CWallet::UnlockAllCoins()
{
    AssertLockHeld(cs_wallet); 
    setLockedCoins.clear();
}
bool CWallet::IsLockedCoin(uint256 hash, unsigned int n) const
{
    AssertLockHeld(cs_wallet); 
    COutPoint outpt(hash, n);
    return (setLockedCoins.count(outpt) > 0);
}
void CWallet::ListLockedCoins(std::vector<COutPoint>& vOutpts)
{
    AssertLockHeld(cs_wallet); 
    for (std::set<COutPoint>::iterator it = setLockedCoins.begin();
         it != setLockedCoins.end(); it++) {
        COutPoint outpt = (*it);
        vOutpts.push_back(outpt);
    }
}
 
class CAffectedKeysVisitor : public boost::static_visitor<void> {