Shut the process down gracefully

[dgaubert]

As per Node.js documentation warms, we should implement a graceful shutdown for the service whenever the process has to deal with uncaughtException or unhandledRejection events or when the process receives OS's signals such as SIGTERM and SIGINT. That means, we need to handle all incoming request before closing the server and liberate all the resources the process may have allocated, then we can exit the process gracefully.

[dgaubert]

cc/ @CartoDB/rt-managers