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.

#include "base58.h"
#include "chain.h"
#include "rpc/server.h"
#include "init.h"
#include "validation.h"
#include "script/script.h"
#include "script/standard.h"
#include "sync.h"
#include "util.h"
#include "utiltime.h"
#include "wallet.h"
#include "merkleblock.h"
#include "core_io.h"
#include <fstream>
#include <stdint.h>
#include <boost/algorithm/string.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>
#include <univalue.h>
#include <boost/assign/list_of.hpp>
#include <boost/foreach.hpp>
using namespace std;
void EnsureWalletIsUnlocked();
bool EnsureWalletIsAvailable(bool avoidException);
std::string static EncodeDumpTime(int64_t nTime) {