Switch license to GPLv3

According to the decision of the GRNET Board of Directors,
switch license to GPLv3.

This commit will be propagated to the release
and master branches based on git flow, and the next
release will be licensed as GPLv3.
mapper: Fix v2 mapdata layout

Introduce new v2 mapdata layout. The new layout introduces restrictions on the
size of the object on the map, but it is simpler and does not split objects
between mapdata blocks. It also behaves correctly with small blocksizes.