XOVER, NEWNEWS, XHDR responses may be arbitrarily long and cause
memory usage via buffering. This problem would exist even if we
were to optimize them by caching headers for fast retrieval and
search.
Introduce a generic API to handle all of these commands fairly
without triggering excessive buffering and unfairness of the
existing implementation.
Generating these responses is still expensive for now, but at least
the implementation is fair to other clients and prevents one client
from using one of these commands to monopolize resources away from
other clients.