Support multi-threaded compression in COW v3

Performance of COW v3 is now on par with v2 in both multi-threaded and
single threaded configurations. Note, v2 cow writer can cache up to 1024
blocks in memory if multi-threaded compression is enabled(even though
batch size is configured as 200). For a fair comparison, benchmarks are
ran with batch size of 256. For batch size of 256 or greater, v2 and v3
have similar multi-threaded performance.

Test: th
Bug: 313962438
Change-Id: I377c8291689a7a038bb00b09d7371a155e6972e9