Add a BandwidthStatistics option and consensus parameter

Stage 1:

We propose adding a new BandwidthStatistics torrc option, which activates reporting of all the bandwidth statistics. At the moment, all statistics are controlled by ExtraInfoStatistics, but we propose using the new BandwidthStatistics option specifically for the bandwidth statistics.

The default value of this option should be 1. (The existing bandwidth statistics are reported by default.)

The new option should have a manual page entry, and test_parseconf.sh tests in src/test/conf_examples.

Stage 2:

Add a BandwidthStatistics consensus paramter.

Change the default value of the BandwidthStatistics option to "auto". If the value is "auto", use the value of the consensus parameter. If the consensus parameter is not set, use "1".

Update the manual page to describe the new consensus parameter.

See the proposal: https://gitweb.torproject.org/torspec.git/tree/proposals/313-relay-ipv6-stats.txt#n298