apt-get clean does not look for lock file

[Milos Radojevic]

I'm not sure if this is a apt or do-release-upgrade bug, but one one of these options produce bug:
1) do-release-upgrade don't create lock file or,
2) apt-get clean ignores lock file

To reproduce error do this:
1. Run do-release-upgrade
2. Wait until files are started fetching
3. do apt-get clean in different session
4. do-release-upgrade is broken.

ProblemType: Bug
DistroRelease: Ubuntu 14.10
Package: apt 1.0.9.2ubuntu2
ProcVersionSignature: Ubuntu 3.13.0-39.66-generic 3.13.11.8
Uname: Linux 3.13.0-39-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.10
Architecture: amd64
CurrentDesktop: Unity
Date: Mon May 11 01:04:17 2015
InstallationDate: Installed on 2014-09-21 (231 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
SourcePackage: apt
UpgradeStatus: Upgraded to utopic on 2015-05-10 (0 days ago)