libsnapshot: Batch write COW operations in a cluster

COW operations are written in cluster. All the COW ops
and the COW data in this cluster are contiguous. Hence,
batch these writes and write them in one syscall.

Writes are done when the cluster is full or when
label ops are written.

OTA install time (without post-install) on Pixel 6 Pro:

          Without-this-patch        With-this-patch

Full OTA: 17 Minutes               13 Minutes

Following are the OTA install times with both the optimization.
viz - batch writes + 2 threads for compression.

OTA install (without post-install) on Pixel 6 Pro.

All numbers are in minutes.

Full ota - 2.2G

Compression   Without-this-patch     With-this-patch
=========================================================
gz            23                      13
lz4           13                       7
none          13                       7

Incremental OTA - 376M

Compression   Without-this-patch     With-this-patch
=========================================================

gz            22                     16
lz4           14                     11
none          15                     11

Bug: 254188450
Test: Full / Incremental OTA on Pixel
Change-Id: Ie3aba1ff28a6569d25a766377efab6cbe78d9277
Signed-off-by: Akilesh Kailash <akailash@google.com>