Straight out the write-marking requirement rules

Currently we DBUG_ASSERT(marked_for_write_or_computed()) for every Field::store_value() however we use these stores even for reading purposes. To override this, the assertion has evolved from write-marking assertion to a complicated set of rules, which in particular includes

• checking for computed - however, we loose a big plane of assertions when writing the computed stored columns
• checking for table->record[0], which is another thing preventing from using an arbitrary record for storing and invoking ha_* functions

For now the proposal is to make two sets of store_value functions: one for read and one for write. Let's see if current use cases may fit into it, and if they do, then implement it.