Problem:
A matrix project is set up to build on several slave nodes.
Its workspace directory is cleaned at slaves even if there is a running build.

Description:
WorkspaceCleanupThread uses shouldBeDeleted method to determine should the
workspace be deleted or not. It checks:
1) was the workspace directory modified at least 30 days ago;
2) was the project last built on the specific node.

Matrix project workspace directory contains axis subdirectories, therefore it is
not touched by a build. So the 1st check is passed.
Matrix project build node points to the master. So the 2nd check is passed for a
slave.
So a matrix project workspace is deleted after 30 days since last axis
configuration change.