MINOR: proxy: yield during servers purge

MINOR: proxy: yield during servers purge

Function proxy_purge_servers() loops through server purge list and tries
to remove them. This is executed under thread isolation for the whole
function duration.

To prevent blocking for too long, yield after 50 servers deleted. The
purge task is immediately requeued but this gives an opportunity for the
other threads to achieve some work.

For now, a static value is applied as the yield limit. It may be useful
to render it configurable if users want to prioritize traffic even more.