Several different test cases were failing under the same reason: the
fields in a vcol expression were not marked during marking columns of a key
contatining virtual column for read.

Fix: make marking columns of a key for read a special case where
register_field_in_read_map() is done instead of plain bitmap_set_bit().

Some test cases are only reproducible in 10.4+, but the fix is applicable
to 10.2+