Implement generic on-modify-request and on-examine-response notifications. We should generalize the HTTP specific notifications for all channels. We can do this simply by implementing them in nsBaseChannel (and nsHttpChannel if we haven't gotten nsHttpChannel to extend from nsBaseChannel yet). We need to think about the "http-on-examine-merged-response" case a little bit more. Perhaps the general use-case is really just an "on-examine-response" message sent before calling OnStartRequest on the channel's stream listener. That would make "on-examine-response" match "http-on-examine-merged-response". We probably do not need a generic version of "http-on-examine-response" as the distinction between that and the other is very HTTP specific. We may also wish to support a prioritization scheme (most likely something like the one that will be used for content sniffers; see bug 329644).