Remove size_t num_elements from start_element_type

AMF is the only format that requires the formatter to know the
number of elements before serialization of those elements.

As we don't support AMF and don't plan to, there is no need for
this in the function call. This removes a bunch of sequential
scans of the tmp tables.

Fixes #74
Remove some unnecessary calls to num_nodes/etc

We don't use the results of the num_nodes/ways/relations calls in
osm_responder::write, so we can just delete the calls.
Remove num_(nodes|ways|relations) completely

We are no longer calling these functions, so we don't need to keep
them around. If we do introduce a new call at some point that needs
this information we can track the size of the data to return more
efficiently with affected_rows.
Merge pull request #75 from pnorman/start_num_elements

Eliminate num_(nodes|ways|relations)