Investigate changes in SERVER-16917: Return fileSize to db.stats() when using wiredTiger

Description

SERVER ticket description: Some properties in db.stats() when using mmap do not exist when querying with wiredTiger configured. However, where a similar stat does exist, it should be included. In this case, the most obvious missing stat is fileSize.

In WiredTiger this is the same as storage size plus index size.

Change Description: This commit adds a new field to the collStats and dbStats commands, both at the cluster level and at the standalone level, and removes an obsolete field from dbStats at both levels.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)