cmd/web: stream large files asynchronously.
cmd/web: stream large files asynchronously.

We had a nice chunkyreader() loop for writing files, but unfortunately,
Tornado captured the full content of those files before writing them to the
client.  Oops.

Change things around so we don't end up buffering some multiple of the
ENTIRE FILE in memory.

Signed-off-by: Avery Pennarun <e03ccfb89d416c4697ad6970ce86a92c1c28a78e@gmail.com>

gevaerts/bup,mhoeher/bup,gevaerts/bup,pombredanne/bup,jbaber/bup,apenwarr/bup,jbaber/bup,mhoeher/bup,pombredanne/bup,pombredanne/bup,mhoeher/bup,tjanez/bup,pombredanne/bup,apenwarr/bup,ToxicFrog/bup,ToxicFrog/bup,pombredanne/bup,gevaerts/bup,tjanez/bup,ToxicFrog/bup,ToxicFrog/bup,tjanez/bup,jbaber/bup,tjanez/bup,pombredanne/bup,jbaber/bup,tjanez/bup,mhoeher/bup,jbaber/bup,ToxicFrog/bup,gevaerts/bup,apenwarr/bup,apenwarr/bup,mhoeher/bup,tjanez/bup,jbaber/bup,gevaerts/bup,mhoeher/bup