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.

class CAccountingEntry;
struct CBlockLocator;
class CKeyPool;
class CMasterKey;
class CScript;
class CWallet;
class CWalletTx;
class uint160;
class uint256;
enum DBErrors
{
    DB_LOAD_OK,
    DB_CORRUPT,
    DB_NONCRITICAL_ERROR,
    DB_TOO_NEW,
    DB_LOAD_FAIL,
    DB_NEED_REWRITE
};
class CHDChain
{
public:
    uint32_t nExternalChainCounter;
    CKeyID masterKeyID; 
    static const int CURRENT_VERSION = 1;
    int nVersion;
    CHDChain() { SetNull(); }