MVStore: support two compression levels; measure read and written bytes; ascending iteration over entry set

Spatial index: a few bugs have been fixed (using spatial constraints in views, transfering geometry objects over TCP/IP).

Issue 368: ON DUPLICATE KEY UPDATE did not work for multi-row inserts. Test case from Angus Macdonald.

MVStore: store root pages first; better size estimation for serialized objects; option to compress database content.

Memory usage calculation should also depend on the memory size of the value, not just on the number of pages.