Minor refactorings

Rename ServerKit classes so that their function is clearer.
Remove some unused code
Fix setting HTTP server flags in the Apache module
Allow customizing the instance registry directory and the data buffer directory.
Set the server_software parameter and the SERVER_SOFTWARE environment variable
Remove integration test for passenger_show_version_in_header

It is now a global option, not per-location.
Introduce passenger_stat_throttle_rate in Nginx, and make Apache's PassengerStatThrottleRate global
Fix some bugs. All Nginx integration tests now pass