Create traffic capture output in subdirectory of dbpath

The TrafficRecorder attempts to use logpath by default. There are two unfortunate side effects of this:

• The logpath must be set even for trivial experiments with traffic capture
• Files are named by the startTrafficCapture command. It can be difficult to collect the set of files in certain situations.

I would like us to follow the ftdc pardigm wherein by default we make a subfolder in dbpath and put files there. This has the added advantage of automatically capturing traffic recordings via resmoke rules where applicable.