Optimize snapshots storage

[ValentaTomas]

Optimize snapshot storage in preparation to enable the automatic pausing of snapshots.

Subtasks:

• Handle empty block in orchestrator (was already implemented)
• Skip empty blocks during snapshotting
  • Memfile (untested)
  • Rootfs (lower priority)
• Skip unchanged blocks during snapshotting
  • Memfile (untested)
• Start creating empty blocks during template building and upload diff with header instead of the full template
  • Memfile (untested)
  • Rootfs (untested)
• Handle diffs instead of just full snapshots in orchestrator for normal sandbox start (right now we handle diffs only for resume)

[linear]

E2B-1423 Optimize snapshots storage

[ValentaTomas]

The remaining work to do here is:

• Test if everything works
• Profile the impact and optimize

[ValentaTomas]

When checking pause on some bigger snapshots (around 3 GB total) the post-processing of memory took most of the time—when we work on the outlines optimizations we should take into account how they might slow down this further+optimize this speed also.

[ValentaTomas]

This PR will probably be cherry picked for the actual changes after we add tests/benchmarks to the code that would be affected. Otherwise this could result in regression.