Remove redundant information in Column

[crwen]

Column need a lot of information that can be found in ColumnDesc. Is there an elegant way to remove these fields?

pub struct Column {
    pub datatype: Datatype,
    pub value: Arc<dyn Any>,
    pub is_nullable: bool,
    pub name: String,
}

Here are some reasons that why we need these fields now:

• Encoding and decoding: datatype is needed to decode from wal, and is_nullable is used to decide whether to cast to Option
• Bindings:
  • Bindings can use name to identify columns. If we remove name, We need to ensure that the order of columns(getting from tonbo) is the same as the order defined by the schema, and we should set name for every column that is to return to user
  • Bindings use datatype to convert the value to the corresponding value, we can use column description though

[linear]

TON-92 Remove redundant information in `Column`