libsnapshot: Do not store CowOperations in memory during writes

The only use case of storing COW operations in memory
was to calculate SHA256 sum during Finalize() - However,
we haven't been doing that since day one of libsnapshot_cow library.

This consumes peak memory ~14mb on an incremental OTA which has 700k+
COW operations writes on one partition.

We can rather use this memory for Async operations where we will have
to cache the buffers.

Bug: 254188450
Test: Incremental OTA
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I2165ed95ff26a9dfb465dc3120df61bb02eee27b