Modify buckets in bucketlist to automatically switch to disk-backed storage when they grow beyond a given size

[graydon]

Buckets should be memory-bounded and switch to disk-backed form when larger than some threshold value (say, 10MB or so?). This will require rewriting the merge algorithm to optionally use disk-based iteration as well, and rewriting the history bucket-writing code to just flip the bucket to disk-backed mode.