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.

"Arguments:\n"
            "1. requests     (array, required) Data to be imported\n"
            "  [     (array of json objects)\n"
            "    {\n"
            "      \"scriptPubKey\": \"<script>\" | { \"address\":\"<address>\" }, (string / json, required) Type of scriptPubKey (string for script, json for address)\n"
            "      \"timestamp\": timestamp | \"now\"                        , (integer / string, required) Creation time of the key in seconds since epoch (Jan 1 1970 GMT),\n"
            "                                                              or the string \"now\" to substitute the current synced blockchain time. The timestamp of the oldest\n"
            "                                                              key will determine how far back blockchain rescans need to begin for missing wallet transactions.\n"
            "                                                              \"now\" can be specified to bypass scanning, for keys which are known to never have been used, and\n"
            "                                                              0 can be specified to scan the entire blockchain.\n"
            "      \"redeemscript\": \"<script>\"                            , (string, optional) Allowed only if the scriptPubKey is a P2SH address or a P2SH scriptPubKey\n"
            "      \"pubkeys\": [\"<pubKey>\", ... ]                         , (array, optional) Array of strings giving pubkeys that must occur in the output or redeemscript\n"
            "      \"keys\": [\"<key>\", ... ]                               , (array, optional) Array of strings giving private keys whose corresponding public keys must occur in the output or redeemscript\n"
            "      \"internal\": <true>                                    , (boolean, optional, default: false) Stating whether matching outputs should be be treated as not incoming payments\n"
            "      \"watchonly\": <true>                                   , (boolean, optional, default: false) Stating whether matching outputs should be considered watched even when they're not spendable, only allowed if keys are empty\n"
            "      \"label\": <label>                                      , (string, optional, default: '') Label to assign to the address (aka account name, for now), only allowed with internal=false\n"
            "    }\n"
            "  ,...\n"
            "  ]\n"
            "2. options                 (json, optional)\n"
            "  {\n"
            "     \"rescan\": <false>,         (boolean, optional, default: true) Stating if should rescan the blockchain after all imports\n"
            "  }\n"
            "\nExamples:\n" +
            HelpExampleCli("importmulti", "'[{ \"scriptPubKey\": { \"address\": \"<my address>\" }, \"timestamp\":1455191478 }, "
                                          "{ \"scriptPubKey\": { \"address\": \"<my 2nd address>\" }, \"label\": \"example 2\", \"timestamp\": 1455191480 }]'") +
            HelpExampleCli("importmulti", "'[{ \"scriptPubKey\": { \"address\": \"<my address>\" }, \"timestamp\":1455191478 }]' '{ \"rescan\": false}'") +
            "\nResponse is an array with the same size as the input that has the execution result :\n"
            "  [{ \"success\": true } , { \"success\": false, \"error\": { \"code\": -1, \"message\": \"Internal Server Error\"} }, ... ]\n");