Push the logic about whether to evict pages with too many deleted items down a level.
Move the READGEN_OLDEST logic into a function to make it less obscure.
Force eviction if we see many consecutive deletes when scanning through a page.
Do the forced eviction call in the lower-level next/prev code.  Now that we may want to force eviction even though we find some useful records, doing it in the general-purpose caller is a pain.
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
Revert changes to flow control that are no longer required.
Merge branch 'develop' into force-evict-many-deletes
Merge pull request #1442 from wiredtiger/force-evict-many-deletes

Force evict a page if we see many deletes while scanning