Hot restart Python wrapper

• SIGTERM: Will cleanly terminate all child processes and exit.
• SIGCHLD: If any of the child processes shut down unexpectedly, the restart script will shut everything down and exit to avoid being in an unexpected state. The controlling process manager should then restart the restarter script to start Envoy again.
• SIGUSR1: Will be forwarded to Envoy as a signal to reopen all access logs. This is used for atomic move and reopen log rotation.