client-store interface

Working on the range-issue in squid-3...

I think we would benefit from a extension to the store client interface,
to support notification of new data arriving (as opposed to the
completion of read requests). This would be used for:
1) multipart responses in the future, where the order of offset-length
data isn't predictable.
2) passing through range responses where the range processing fail for
whatever reason.

Without this, we'd need an ugly read-any-offset style API to be
reintroduced, like we had before Adrian straightened out the pipeline
core.

The concept for use is:
if (don't know what offset we need)
  {
    store_client->notify_on_data();
  }
else
  {
    store_client->read();
  }

Thoughts? Objections?

Rob

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