snapuserd: Sort REPLACE ops for batch merge

Since we will be iterating forward for user-space
merge, we need to sort the blocks in increasing order
so that blocks can be batch merged if contiguous.

For dm-snapshot merging, we will continue to sort
in decreasing order.

Bug: 193863397
Test: Snapuserd_test
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I25fb5fce054f716a2ad0dddc0d0c3afef18bc7ad