[tor-relays] Disable CellStatistics !!!

[starlight.2017q4 at binnacle.cx]

After many crashes and much pain, I determined that
having CellStatistics enabled causes a busy relay
to consume at least two or three _gigabytes_ of
additional memory.  Relay operators with less than
16GB per instance are advised to disable it.

By default CellStatistics is disabled unless explicitly
set in torrc.

CellStatistics can be turned off without restarting,
via the control-channel command

   setconf CellStatistics=0