libsnapshot:snapuserd: Multithreading support

Add worker threads per partition to serve the IO request.

Remove memset of buffer in IO path which was impacting
4k IO performance.

update_verifier performance:

1: ~10-12 seconds with this change (both on full OTA and incremental
OTA); ~70 seconds observed without this changeset

2: ~8 seconds without the daemon once merge is completed
and snapshot devices are removed.

Bug: 181293939
Test: update_verifier, full OTA, incremental OTA
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Id90887f3f4a664ee5d39433715d1c166acbd6c60