Allow to detect whether field missing or null

[laskoviymishka]

When binlog enabled in noblob or minimal mode there is no tool to detect whether field setted nil on purpose or it null becouse of blob simply ignored.

However protocol has support for that. I propose to add

SkippedColumns [][]int

In each row update event and populate it for every column in row which has no isBitSet(bitmap, i)