venti:
* stop storing unused wtime, train fields in IEntry.
* prefetch arena tocs more gradually; set icacheprefetch=1
* yank out dead lump readahead code
* yank out dead dcache readahead code
* fix sync bug reported by anothy et al.
* new, cleaner icache code

mirrorarenas:
* fix thread-stack access race (wsync)
* new flag -s disables SHA1 checks, useful if destination is remote

verifyarena:
* accept venti-standard file:range syntax