Don't show specific error messages when proxying fails.

The error messages contain the IP address of a proxy or client. I don't
know how to prevent the lower-level error messages from including the IP
address, so just omit the whole message.

The error messages look like
error copying WebSocket to ORPort: read tcp X.X.X.X:YYYY: connection reset by peer
error copying WebSocket to ORPort: read tcp X.X.X.X:YYYY: use of closed network connection
The great majority of them are "use of closed network connection", which
may be a bug related to the sharing of streams in different goroutines.
Here are the frequencies of different errors:
      1 connection timed out
     19 connection reset by peer
   1137 use of closed network connection
Factor out build_query_string.
Send polling interval to proxies.

Include a check "CHECK-BACK-IN" parameter in response to proxy requests,
that specifies when the proxy should contact the facilitator again.
Currently this value is hard coded.
Return polling interval in get_reg().

Check that the polling interval reveived from the facilitator is a
numeric value and return it to the caller, along with client and relay
specs.
Get polling interval from facilitator.

Set the polling interval to be the value of the "check-back-in"
parameter from facilitator responses. If the polling interval received
is invalid, print an error message and keep polling at the previous
rate.
Increase default and minimum polling intervals.

Increase DEFAULT_FACILITATOR_POLL_INTERVAL to 3600 seconds and increase
MIN_FACILITATOR_POLL_INTERVAL to 10 seconds.
Make facilitator_poll_interval option override other settings.

If the facilitator_poll_interval option is provided, and is a valid
value, it overrides the facilitator-provided interval and the default
interval. If the option is missing, fall back on the facilitator
interval, and absent that, the default interval. If any value is smaller
than MIN_FACILITATOR_POLL_INTERVAL, that is used instead.
Do not fail if facilitator doesn't provide a polling interval.

Do not raise an exception in get_reg() if the facilitator doesn't
provide a polling interval.
Extract proxy requests for graphing with R.
Extract client requests for graphing with R.
Ignore socket.error in setting IPV6_V6ONLY.
Remove unused pt_enverror.
Change unused pt_smethoderror to pt_cmethoderror.
Tolerate failures in opening listeners.

Carry on as long as we have at least one local and one remote listener.  In
reproduce it).
Remove nonexistent version 0.13 from ChangeLog.
Increase VERSION to 0.12.
ChangeLog for #8319.
Remove superfluous undefined default value.
Poll interval message.
Rewrite MIN_FACILITATOR_POLL_INTERVAL test and move out of else.