closes #496 Add index access stats and a test
fixes #501 Add richer index access statistics, including nscanned,
nscannedObjects, inserts, and deletes. No updates yet - those are not as
trivial to track because of the way env->update_multiple generates keys
(you cannot immediately tell that two keys have changed based on the way
the DBTArray's were populated)

Conflicts:

	src/mongo/db/namespace_details.cpp
Fix stats