MCOL-4600 CAST(decimal AS SIGNED/UNSIGNED) returns a wrong result

The "SIGNED" part of the problem was previously fixed by MCOL-4640.
Fixing the "UNSIGNED" part.

- Adding TDecimal64::toUInt64Round() and Decimal::decimal64ToUInt64Round()
- Renaming Decimal::narrowRound() to decimal64ToSInt64Round(),
   for a more self-descriptive name, and for symmetry with decimal64ToUInt64Round()
- Reusing TDecimal64::toSInt64Round() inside decimal64ToSInt64Round().
  This change was forgotten in MCOL-4640 :(
- Removing the old code in Func_cast_unsigned::getUintVal with pow().
  It caused precision loss, hence the bug. Adding a call for
  Decimal::decimal64ToUInt64Round() instead.
- Adding tests for both SIGNED and UNSIGNED casts.

Additional change:
- Moving the wide-decimal-to-uint64_t rounding code from Func_cast_unsigned::getUintVal()
  to TDecimal128::toUInt64Round() (with refactoring). Adding TDecimal::toUInt64Round()
  for symmetry with TDecimal::toSInt64Round(). It will be easier to reuse the code
  with way.

utils/funcexp/func_bitwise.cpp

@@ -100,7 +100,7 @@ datatypes::TUInt64Null DecimalToBitOperand(Row&  row,
         return ConvertToBitOperand<int128_t>(val);
     }
 
-    return datatypes::TUInt64Null((uint64_t) d.narrowRound());
+    return datatypes::TUInt64Null((uint64_t) d.decimal64ToSInt64Round());
 }