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 "wallet/wallet.h"
#include "base58.h"
#include "checkpoints.h"
#include "chain.h"
#include "wallet/coincontrol.h"
#include "consensus/consensus.h"
#include "consensus/validation.h"
#include "key.h"
#include "keystore.h"
#include "validation.h"
#include "net.h"
#include "policy/policy.h"
#include "primitives/block.h"
#include "primitives/transaction.h"
#include "script/script.h"
#include "script/sign.h"
#include "timedata.h"
#include "txmempool.h"
#include "util.h"
#include "ui_interface.h"
#include "utilmoneystr.h"
#include <assert.h>
#include <boost/algorithm/string/replace.hpp>