Properly support decimal types in mysql row -> arrow conversion

[Sevenannn]

Properly support decimal types in mysql row -> arrow conversion. Changes include:

• Use column.decimals() to infer mysql decimal precision in rows_to_arrow
• Map ColumnType::MYSQL_TYPE_DECIMAL | ColumnType::MYSQL_TYPE_NEWDECIMAL into concrete Arrow Decimal types instead of None in map_column_to_data_type
• Use non-empty Array builder for mysql ColumnType::MYSQL_TYPE_DECIMAL | ColumnType::MYSQL_TYPE_NEWDECIMAL

[phillipleblanc]

This could use a test, but otherwise LGTM.