libre office: only one conversion job at the same time (locking)

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@3365 1f8fe7d8-47f0-464c-8d0a-336f4953ab05
build auto version (2830) ...

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@3366 1f8fe7d8-47f0-464c-8d0a-336f4953ab05