oracle - fix returning for non-integer types

Now returning for inserts works just like postgres. I had to split the query into two separate SQL commands (insert and select) because directly returning the values requires to know the type beforehand. Before this it worked fine for integer types (which is the usual auto_increment replacement) but not for other types (e.g. char).

The problem came up while using bookshelf with a uuid as key and a forced insert which uses `insert(<values>, <idAttribute>)` by default. I think bookshelf should omit the returning <idAttribute> if the idAttribute is already specified in the <values>.