wallet: Treat `mnemonichdchain` records as key material

This is a temporary change to ensure that if this record is unreadable,
we immediately shut down the node and don't make any further changes
that could impair our ability to recover from this state later.

Part of zcash/zcash#5806.

src/wallet/walletdb.cpp

@@ -909,6 +909,7 @@ static bool IsKeyType(string strType)
     return (strType== "key" || strType == "wkey" ||
             strType == "hdseed" || strType == "chdseed" ||
             strType == "mnemonicphrase" || strType == "cmnemonicphrase" ||
+            strType == "mnemonichdchain" ||
             strType == "zkey" || strType == "czkey" ||
             strType == "sapzkey" || strType == "csapzkey" ||
             strType == "vkey" || strType == "sapextfvk" ||