snapuserd: Move dm-snapshot-merge related code

Move all the snapuserd files related to dm-snapshot/dm-snapshot-merge
to a seperate directory. We will have a logical seperation
to maintain user-space merge code.

No code changes other than moving files around.

Bug: 193863397
Test: Build
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Ide0d824bb19f9d870ec32699d0a37de0e5d6ce81