Add -zapwallettxes cli/config option, used for wallet recovery

This diagnostic tool removes all "tx" records from the wallet db,
then forces a full rescan, to rebuild "tx" records accurately.

src/wallet.cpp

@@ -1497,6 +1497,30 @@ DBErrors CWallet::LoadWallet(bool& fFirstRunRet)
 }
 
 
+DBErrors CWallet::ZapWalletTx()
+{
+    if (!fFileBacked)
+        return DB_LOAD_OK;
+    DBErrors nZapWalletTxRet = CWalletDB(strWalletFile,"cr+").ZapWalletTx(this);
+    if (nZapWalletTxRet == DB_NEED_REWRITE)
+    {
+        if (CDB::Rewrite(strWalletFile, "\x04pool"))
+        {
+            LOCK(cs_wallet);
+            setKeyPool.clear();
+            // Note: can't top-up keypool here, because wallet is locked.
+            // User will be prompted to unlock wallet the next operation
+            // the requires a new key.
+        }
+    }
+
+    if (nZapWalletTxRet != DB_LOAD_OK)
+        return nZapWalletTxRet;
+
+    return DB_LOAD_OK;
+}
+
+
 bool CWallet::SetAddressBook(const CTxDestination& address, const string& strName, const string& strPurpose)
 {
     AssertLockHeld(cs_wallet); // mapAddressBook