Cache buffer for MDC

When using MEG to replay an entire MDC cycle, the cache should be populated in advance with all MDC events. This is to avoid delays due to event download from graced-playground, which could bias the event upload time. The caching of all MDC events occupies approximately 100 GB, which could be too much for e.g. a laptop.

It would be nice to have a circular buffer cache containing just a subset of events, let's say a day, ready to be uploaded.