Add X-Forwarded-For to access log records

https://github.com/cloudfoundry/gorouter/issues/48
Namespace x-forwarded-for, add test for when missing

 - Added an explicit key so that log parsers can easily see which column
   corresponds to x-forwarded-for, especially since it might be empty
 - Tested handling of missing values
 - Also extended missing value test to test more existing
   things-which-can-be-nil

[#76123118]

Signed-off-by: Michael Fraenkel <fraenkel@us.ibm.com>
In config's Process(), assert that "StartResponseDelayInterval" cannot be
greater than "DropletStaleThreshold" for avoiding possible routing
failure.
Slightly nicer warning message

when automatically changing DropletStaleThreshold to StartResponseDelayInterval

Signed-off-by: Michael Fraenkel <fraenkel@us.ibm.com>