Unescaping re-added, support limited to schema introspection (#2850 will bring more complete coverage)

@morozov review: According to MySqlPlatform::getListTableColumnsSQL(), the default column is always selected. Removed ??

Serial fields already have a default value. By opting out of `notnull`
serial fields will just set the next value in the sequence.

I know, this is REALLY nasty but that was the only way I've found to
make it work properly.

And then we don't need to wait till v3 to not require comments on the
JsonType.