In 0.2.3.8-alpha we attempted to "completely disable stats if we aren't
running as a relay", but instead disabled them only if we aren't running
as a server.
This commit leaves DirReqStatistics enabled on both relays and bridges,
and disables (Cell,Entry,ExitPort)Statistics on bridges.
These stats are currently discarded, but we might as well
hard-disable them on bridges, to be clean.
Fix for bug 5824; bugfix on 0.2.1.17-rc.
Patch originally by Karsten Loesing.
