some bug fixes that need merging into earlier patches
[dm-bufio] make __scan honour max_age_seconds
[dm-transaction-manager] add support for prefetching blocks of metadata
[dm-thin] Prefetch metadata blocks that were not in the cache when doing lookup
[dm-thin-metadata] Change dm_thin_find_block() to allow blocking, but not issuing IO.
[dm-thin] An attempt at throttling incoming IO based on the time it's taking the worker to do one loop.

I'm really not sure this helps at all.
[dm-thin] remove a comment

Merge with the prefetch patch