multi-update was setting up read_set/vcol_set in
multi_update::initialize_tables() that is invoked after
the optimizer (JOIN::optimize_inner()). But some rows - if they're from
const tables - will be read already in the optimizer, and these rows
will not have all necessary column/vcol values.

* multi_update::initialize_tables() uses results from the optimizer
  and cannot be moved to be called earlier.
* multi_update::prepare() is called before the optimizer, but
  it cannot set up read_set/vcol_set, because the optimizer
  might reset them (see SELECT_LEX::update_used_tables()).

As a fix I've added a new method, select_result::prepare_to_read_rows(),
it's called from inside the optimizer just before make_join_statistics().

This fixes a bug where handler::read_range_first (for example)
needed to compare vcol values that were not calculated yet.

As a bonus it fixes few cases where vcols were calculated twice

sql_insert.cc calls handler->ha_update_row() for
REPLACE and INSERT... ON DUPLICATE KEY UPDATE

cannot use TABLE:merge_keys for that, because Field::part_of_key
was designed to mark fields for KEY_READ, so a field is not a
"part of key", if only prefix of the field is.

* don't issue an error for ER_KEY_BASED_ON_GENERATED_VIRTUAL_COLUMN
* support keyread on vcols
* callback into the server to compute vcol values from mi_check/mi_repair
* DMLs just work. Automatically.

update tests and results, fix bugs

will be used later by storage engines (e.g. in MyISAM repair or in
InnoDB purge)

because it's only doing to show the table structure, not look
at the data.

always set them according to the repair method used

and declare few other Field getters to be 'const'

Make update_virtual_fields() a method of TABLE, to be consistent
with TABLE::update_default_fields().

TABLE::mark_virtual_col() was polluting table->vcol_set
and that confused the following mark_virtual_col()

make it to work when read_set == vcol_set, that is, when
the caller wants to get a one complete bitmap of all fields
a particular vcol depends on.

it was done only in some access methods, not in all,
so the caller had to update_virtual_fields anyway.

In spatial indexes there can be only one keyseg.
Make it explicit in the code, don't pretend that this can work
with the arbitrary number of keysegs.

and FIELDFLAG_xxx constants

prefer x->field over table->field[x->field->field_index]

only use my_print_error when the error message is not
ER(error_code)

the stage is changed from 'old_stage' to 'new_stage',
not the other way around

and partition_info::set_show_version_string - they were
already broken and impossible to maintain

unused freshly initialized record should be trashed
with TRASH_ALLOC, not TRASH_FREE

remove redundant method