snapuserd: snapshot-merge optimizations

1: Cap the iou-wq threads to 1 - This will cut down the CPU usage and
   not spin up new threads

2: Add new ring initialization flags

Bug: 425765067
Test: OTA on Pixel
1: Verify no more additional threads are spin up during merge
2: snapshot-merge time reduces by ~30 seconds when every block is a copy
   block.
Change-Id: I51d34070ae754c8235405819efc43ea1425d69e2
Signed-off-by: Akilesh Kailash <akailash@google.com>