nntp: improve fairness during XOVER and similar commands

nntp: improve fairness during XOVER and similar commands

For other commands generating long responses, we generally want
to yield to another client after emitting 100 .  However,
XOVER-based responses already query 200 lines worth of responses
at a time, so we were sending 20000 lines before yielding to
other clients.  This may help avoid timeouts for some clients.