[RFC][PATCH]Speed improvement on the http transport: add keepalive support

[Johan Rydberg]

Martin Pool <mbp at sourcefrog.net> writes:

> PyCurl does a few useful things
>
>  * dns caching
>  * keepalive, including to proxies
>  * future potential for overlapped requests and within-request progress
> indications
>  * apparently does this well on Windows too

I have not looked at PyCurl yet, but is it possible to set generic
headers in the requests?  We need that for range accesses.

~j