Support batch writes for non-compressed ops

This also improves atomicity of ops. If a single Add*Blocks() call
with 100 blocks failed in the middle, partially written blocks would be
discarded, and op count on disk stays unchanged. Previously wew ould
update the op count on disk with partially written blocks, causing
labels to be inaccurate.

Test: th
Bug: 313962438
Change-Id: If175a705f6ec46c1b25c52d0d9f02f01a540ce55