[MERGE] More pack supporting transport fixes

[Martin Pool]

Martin Pool has voted resubmit.
Status is now: Resubmit
Comment:
It looks like the code that does the expansion is a pure function based 
on only the requested offsets and the estimated page size.  I think it 
would be cleaner to split that out to a method on Transport that only 
does the expansion, and call that from readv.  You can then test 
directly that the new method has exactly the behavior you expect for 
particular page sizes and read requests.

I would say you now have an api break that plugins should now override 
_readv not readv, unless they really know what they're doing.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C1188188653.9252.86.camel%40lifeless-64%3E