SERVER-13635 Don't assert that BSONObjs aren't owned

Storage engines are allowed to always return owned BSON.
SERVER-13635 Stop assuming storageSize == cappedSize
SERVER-13635 Only run OplogStart that depend on Extents with mmapv1
SERVER-13635 stop relying on system.indexes in dbtests
SERVER-13635 Destroy OplogStart stage before the RecoveryUnit