libsnapshot: Integrate with snapuserd.

This integrates libsnapshot with dm-user and snapuserd. Tests progress
significantly further now. Tests involving merging still fail as
snapuserd doesn't support this yet.

Bug: 168554689
Test: vts_libsnapshot_test
Change-Id: I464b683b464fe29a646f0f2823b7f4434a878614