I need a long-running private Ethereum net (at least for 2 weeks) with fast block times. I have a single machine which is mining. When setting the difficulty very low, the block-time is very fast in the beginning. "Unfortunately" the difficulty gets adjusted so that after some hours the block-time gets towards the usual 14s. Is there a way to change that? I assume I can re-build geth and set the target block-time somewhere? Or is there any easier method?