added more memory diagnostics for memory usage:
- collection.figures() now returns a "readCache" attribute which contains the
number of and memory usage of entries in the document revisions cache for the
collection, and a "revisions" attribute with the number and memory usage
of entries in the storage engine's revision lookup table for the collection
- the default value for --database.revision-cache-target-size was changed from
75% of RAM to 30% of RAM
