Several optimization to reduce outputobject size

[kleunen]

Several optimizations to reduce size of OutputObject:

• Use OsmID instead of handle to lookup generated geometry (OsmID should be 42 bits now)
• Use non-owning reference in tileindex and remove reference count from output object
• Use intrusive pointer for attribute store reference
• Prevent a temporary created in the mmap file in osm store

[systemed]

10% reduction in total RAM used when converting Great Britain with no discernible impact on runtime - that's excellent! Thanks as ever.