Each collection's query plan cache for is currently cleared after 1000 write operations are issued against the collection. This gives distribution changes a chance to affect planning decisions for query shapes that are already cached.

However, clearing the entire plan cache is aggressive, and at worst will cause deployments with a heavy write load to effectively not use the query plan cache at all.

This mechanism will be made obsolete by SERVER-15225. When SERVER-15225 is resolved, we should remove it entirely.