Basecfg version

@michaelcahill, @agorrod, here's a first attempt at adding versioning and upgrade support to the base configuration file, reference #1244.

Would you please take a look and then let's talk it over?
It's probably not necessary now, but I think it'd be better to auto-generate this via the ```api_config.py``` script. That way we are less likely to re-introduce old settings, which will no longer be allowed.