libsnapshot: Implement CowWriterV3:EmitBlocks

Add in replace ops to WriterV3 without compression support (will be
added in later). CowReader also has to be changed since we no longer
have a CowFooter (our checks need to be updated correspondingly).
We need next_data_pos_ to be pointed to after the operation buffer. This
section is appended to incrementally as we write replace + xor
operations

Test: cow_api_test
Change-Id: Ie979c72f842edd04337d900fd43dac8031207517