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 (request.fHelp || request.params.size() < 2 || request.params.size() > 2)
    {
        string msg = "createmultisig nrequired [\"key\",...]\n"
            "\nCreates a multi-signature address with n signature of m keys required.\n"
            "It returns a json object with the address and redeemScript.\n"
            "\nArguments:\n"
            "1. nrequired      (numeric, required) The number of required signatures out of the n keys or addresses.\n"
            "2. \"keys\"       (string, required) A json array of keys which are bitcoin addresses or hex-encoded public keys\n"
            "     [\n"
            "       \"key\"    (string) bitcoin address or hex-encoded public key\n"
            "       ,...\n"
            "     ]\n"
            "\nResult:\n"
            "{\n"
            "  \"address\":\"multisigaddress\",  (string) The value of the new multisig address.\n"
            "  \"redeemScript\":\"script\"       (string) The string value of the hex-encoded redemption script.\n"
            "}\n"
            "\nExamples:\n"
            "\nCreate a multisig address from 2 addresses\n"