Enable hidden-service statistics by default

With HiddenServiceStatistics 0|1 merged in 0.2.6 which collects two stats as of now (https://research.torproject.org/techreports/extrapolating-hidserv-stats-2015-01-31.pdf), the question is now should we enable them by default in order to have a larger amount of relay reporting those statistics?

Metrics is going to publish the results soon: https://metrics.torproject.org/hidserv-data.html

Keep in mind that this ticket is only about the two current statistics explained in the report linked above. Future experimental statistics could use a second option to enable "experimental stats" instead of the current torrc option that is considered to enable the stable-safe ones (see child ticket for that).