Add a forced eviction check to look for excessively large skip lists.

Large skip lists can cause performance problems - treat a large skip list as though it's grown the page too large.

This helps with the performance degradation seen in:
https://jira.mongodb.org/browse/SERVER-16296

I'm not sure how much I like this solution, and there are some magic numbers in there that should be thought about carefully before deciding to merge (and possibly exposing as configurable).