libsnapshot: v3 structural changes

Changes to structure of v3 header + operation needed for variable block
size. Seperating this CL from the variable block size one so we can get
v3 enabled on cuttlefish

the op count type changes are so that op count matches the type of
max_blocks. Max_blocks is used when op buffer size is not set -> we
default to upper bound of one operation per block in the partition.

Test: th
Bug: 307452468
Change-Id: I1a2581763a4fd6be5d5795f7e4781023e9984256