Maintain the count of deleted items in the cursor until it leaves the
page, check it for forced eviction when we're about to switch pages.
Don't clear the count of deleted items on every iteration, clear it
when the cursor changes shape.
Check whether to evict pages with many deletes where:

1. the scan doesn't continue after finding an item to delete; or
2. a page consists of all deleted items, but less than the threshold.
Merge pull request #1444 from wiredtiger/force-evict-many-deletes-keith

Force evict many deletes