Improve caching of small updates to large values

When an on-disk page grows larger than the configured in-memory maximum, any update to that page will immediate trigger forced eviction. With default MongoDB settings, this behavior makes updates to documents larger than 10MB slow and unpredictable.

Change the trigger for forced eviction so that the page image size is excluded, and when making modifications to a large value on a page, be careful about making full copies of the document in cache (which will trigger forced eviction for large documents).