Certain instances of ParticleRendering, e.g. created by class CopyPasteBlocks, doesn't time out.
Since the class DefaultParticleRenderingRepository uses DurationRepository as backend (issue #726), the duration repository must support storage of durations which never expire.
Even if it kind of goes against the purpose of the duration repository.

Solution: durations with initial value = -1 will never expire.

Resolved with commit 680125e.