When dblink is looping over files for merge/unmerge, temporarily yield to the

When dblink is looping over files for merge/unmerge, temporarily yield to the
scheduler each time a fixed number of files are processed (currently 20).
This gives the scheduler an opportunity to service pending poll events. This
is implemented with a new PollScheduler._schedule_yield() method which calls
poll() exactly once, without blocking, and any services any resulting poll
events.

svn path=/main/trunk/; revision=11116