Implement node churn and uptime statistics

I spent some time this summer designing a schema to support tracking of relay uptime and churn statistics. The relay churn statistic should be split up by platform, version, and guard/exit status for a more fine-tuned insight into the network. The uptime statistic should be split into guard/exit status and version, as it only sees individual platforms. Also, the data the query returns currently is good for a time graph (similar to Karsten's Windows relay uptime graph), but it could be portrayed as a box-plot distribution.

Relay churn is calculated from the unique routers from one week/month/year that appear in the following week/month/year, and is relatively straightforward to calculate. However, this query could use some optimization because it takes a very long time to group individual routers by the times they appear.

Relay uptime is more difficult to calculate with a database query because "uptime sessions" need to be calculated in order to get a correct average. This is near impossible to do with a database query, and must be done programatically (with cursors in pl/pgsql or elsewhere).

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Username: kjbbb