Basic statistics from Anonymous download test

[synctext]

Add the STATS message. Tor collects numerous metrics and we want to support a subset of this, for instance:

Currently no mechanism exists to understand proxy failures and measure their reliability or performance. Instead of blindly working to optimize performance we need some basic stats. This STATS message should facilitate a minimal level of debugging. Critical point is not to leak any privacy sensitive information that could be used to attack anonymity.

Minimal proposed counters for inclusion:

• Total number of bytes relayed for each of the three traffic types (downloaded anonymously, relayed for others, exit traffic with swarm) split in upload and download.
• Online time, indicating how many seconds ago this node is alive (reliability)

Known weakness: by hammering a proxy with 1000 stats requests per second it would be easier to classify the position of a proxy in circuits (entry/relay/exit).

Related issues:
#39 Live stats package
#119 anon-test