mutt: update read progress counter less frequently
mutt: update read progress counter less frequently

As advised in the perf-tuning section of the manual, bump this from 10
to 1000 so that we spend less time updating the UI. No idea if this
makes a measurable difference or not on a modern laptop, but doesn't
hurt to try.

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent