Ensure that a dyno shutdown does not result in early exits

On a code push or dyno restart, a SIGTERM is sent to all pids. This PR adds traps for sigterm to stunnel and pgbouncer to avoid early exits of those processes. A patch to the source of pgbouncer is also included, as it was still exiting prematurely. SIGINT is used to clean up pgbouncer and stunnel after the application has been detected as exiting. 

Also, stunnel has been updated to version 5.00