Introduce the `itersize` parameters for server-side cursors as member variable of the PostGISDatabase class
The itersize parameter, defining the number of datasets transmitted in one go using a server-side cursor, should be adjustable to users of tools like exposure-share. It is dependent on the size of datasets and the memory available of the client-side computer. Therefore, the itersize should be set at the creation of the PostGISDatabase object and not be defined at every call of the execute_server_side function as otherwise the itersize value has to be transmitted through a chain of functions as optional parameter.