Enable logging connection stats for slow requests.

- Add  a 'connection_stats_logfile' option to configure the filename
  used for logging connection statistics for "long" requests.  If not
  specified, no logging occurs.

- Add a 'connection_stats_threshhold' option for configuring a
  "long request" threshold.  If the logfile is specified, and the request
  takes longer than the threshold, log the method, URI, connection loads,
  and connection stores to the logfile in CSV format.

N.B.:  the stats gathering is *not* compatible with the 'perfmetrics'
       middleware (which zeroes out the loads / stores before we can
       log it).
Match the filename used in the ini file
1.25
Merge branch 'tseaver-log_connection_stats'

Conflicts:
	CHANGES.txt
1.24