Move birthday optimization out of ScanForWalletTransactions

This change has no effect on wallet behavior.

On wallet startup, the transaction scan avoids reading any blocks with
timestamps older than the wallet birthday (less than nTimeFirstKey -
TIMESTAMP_WINDOW). This block skipping code currently resides in
CWallet::ScanForWalletTransactions but it doesn't really belong there because
it makes the implementation unnecessarily fragile and hard to understand, and
it never has any effect except at startup (because all other callers do their
rescans based on timestamps other than, but always greater or equal to,
nTimeFirstKey).

src/wallet/wallet.cpp

@@ -1479,11 +1479,6 @@ CBlockIndex* CWallet::ScanForWalletTransactions(CBlockIndex* pindexStart, bool f
         fAbortRescan = false;
         fScanningWallet = true;
 
-        // no need to read and scan block, if block was created before
-        // our wallet birthday (as adjusted for block time variability)
-        while (pindex && nTimeFirstKey && (pindex->GetBlockTime() < (nTimeFirstKey - TIMESTAMP_WINDOW)))
-            pindex = chainActive.Next(pindex);
-
         ShowProgress(_("Rescanning..."), 0); // show rescan progress in GUI as dialog or on splashscreen, if -rescan on startup
         double dProgressStart = GuessVerificationProgress(chainParams.TxData(), pindex);
         double dProgressTip = GuessVerificationProgress(chainParams.TxData(), chainActive.Tip());
@@ -3880,6 +3875,13 @@ CWallet* CWallet::CreateWalletFromFile(const std::string walletFile)
 
         uiInterface.InitMessage(_("Rescanning..."));
         LogPrintf("Rescanning last %i blocks (from block %i)...\n", chainActive.Height() - pindexRescan->nHeight, pindexRescan->nHeight);
+
+        // No need to read and scan block if block was created before
+        // our wallet birthday (as adjusted for block time variability)
+        while (pindexRescan && walletInstance->nTimeFirstKey && (pindexRescan->GetBlockTime() < (walletInstance->nTimeFirstKey - TIMESTAMP_WINDOW))) {
+            pindexRescan = chainActive.Next(pindexRescan);
+        }
+
         nStart = GetTimeMillis();
         walletInstance->ScanForWalletTransactions(pindexRescan, true);
         LogPrintf(" rescan      %15dms\n", GetTimeMillis() - nStart);