Progress reporting from reads via ra_serf

Erik, as mentioned on IRC, here's a patch (untested) which experiments
with injecting invocation of data transfer progress reporting callback
into the read operations performed by ra_serf.

Now, this scheme will report progress for every HTTP response we read,
which may not be exactly what we're looking for (if logical SVN
operations may span multiple HTTP requests).

Anyhow, I think this very rough patch captures the gist of how to
inject this sort of callback using Serf.

- Dan