Implement disk cache for large split packet reassembly

[dktapps]

The current max split size is problematic for Minecraft usage due to extremely oversized skins. It's also problematic to increase it due to exposing servers to denial-of-service attacks.

A workaround for this would be to cache parts of very large split packets (above a certain threshold) on disk temporarily, so that RAM doesn't get exhausted trying to reassemble huge splits for many sessions at the same time. Since disk space is typically more abundant than RAM, this would allow a higher limit on split count, which would alleviate some issues.