Statistics log

Michael, here are the changes to create a statistics logging thread.

The things I think are worth review:

1. The commit b64190ecc736bbd98276087ce586e1e7f61cfec3: this change was because I needed to create a default configuration string value that had whitespace in it, and so I needed to put quotes around it when putting it into the config_def.c file.   I am not sure at all that I put this change in the right place, this one needs to be checked pretty carefully.

2. The commit 7501ecc748ff26ab577b80f4c9c9a49c9bcce6b0: this change is because when we search subcategories, if we "process" each intermediate value, the value type can change (ITEM_ID types get reset to ITEM_NUM).  Essentially, this means booleans don't work if they're in subcategories.

3. I gave the statistics logging thread its own session, that looked right to me but I wanted to mention it.   In general, it's probably worth looking at how I created the statistics logging thread, I may have misunderstood how that's supposed to work.

4. As far as I know, there's no way to say "did the user configure this subcategory at all", so I had to tuck a boolean into the statistics_log subcategory that always has to be set if you want to configure statistics logging.   If there's a way to figure that out, then I could get rid of that extra boolean.