libsnapshot: Add single threaded compression to v3 writer

Add compression path back into Cow operations. Main change is that the
compression algorithm is stored in the header instead of each individaul
op. Have the writer_v3 set this algorithm when parsing options.

There looks to be a lot of code we'll be able to factor out into the
base class, but we can leave that to a later CL.

Test: cow_api_test
Change-Id: Ie9a8eceb5fbdaecae50911119c75f2e51d776a28