Entity field queries need to read entity base table schemas to cast correctly

This seems to impact alternative storage controllers only, so I don't see how we could test it. Thus, RTBC.

I know for sure that MySQL can't use the index on a varchar column when comparing to an int. So says the manual: For comparisons of a string column with a number, MySQL cannot use an index on the column to look up the value quickly. I do not know whether the optimizer has the brains to optimize out the comparisons of a number column with a string, in this case to cast vid only once.

I am somewhat curious why vid is not int in the first place but that's a followup as both pgsql and sqlite has PDO::ATTR_STRINGIFY_FETCHES => TRUE, so that's quite deliberate.