MINOR: cache: Register the cache as a data filter only if response is cacheable

MINOR: cache: Register the cache as a data filter only if response is cacheable

Instead of calling register_data_filter() when the stream analyze starts, we now
call it when we are sure the response is cacheable. It is done in the
http_headers callback, just before the body analyzis, and only if the headers
was already been cached. And during the body analyzis, if an error occurred or
if the response is too big, we unregistered the cache immediatly.

This patch may be backported in 1.8. It is not a bug but a significant
improvement.