Always use Firecracker Diff snapshots (fast, only changed pages) and
merge diff files at the file level when the generation cap is reached.
The previous approach used Firecracker's Full snapshot type which dumps
all memory to disk and can timeout, losing all snapshot data on failure.
Add snapshot.MergeDiffs() which reads each block from the appropriate
generation's diff file via the header mapping and writes them into a
single consolidated file with a fresh generation-0 header.
pptx704
8f06fc554a