Delay loading all DATA sections of the blend file until they're needed.
Loading all data-blocks caused high peak memory usage especially with
libraries - since a lot of data may exist which isn't used directly.
In one test (spring project: 10_010_A.anim.blend),
peaked at ~12.5gig, dropping back to ~2.5gig once loaded.
With this change peaks memory usage reaches ~2.7gig while loading.
Besides this there are some minor gains from not having to read data
from the file-system and we can skip an alloc + memcpy reading data
written with the same version of Blender.
358e07f447