The distinct count result of bitmap/hll column may be incorrect in the spark load mode. Fix some bugs in spark load to solve the above problem. 1. FE is big end but BE is little end. BitmapValues should be transfered to little end in FE's serialization 2. BitmapUnionAggregator/HllUnionAggregator ignore `null` value 3. Make sure encodeVarint64 in FE is consistent with BE Co-authored-by: weixiang <weixiang06@meituan.com>