Catch SIGTERM for graceful shutdown

[Rillke]

Shut down database connection and exit the node process
when SIGTERM is encountered. This is especially important
when nodejs is run as PID1, e.g. in a docker container.

Shutting down connections to clients (browsers) is beyond
this patche's scope.

Resolves #3265

[Rillke]

Note this also affects "normal" non-docker installations: When SIGTERM is received, 2 additional log lines display: "graceful shutdown ..." and "db successfully closed." Though, I think it is generally a good idea to perform graceful shutdowns, closing connections and writing buffers to disk.

I am by no means a UNIX or Etherpad professional so please double-check.

[Rillke]

Agreeing to and signing Apache Contributor license.

[lpagliari]

Thanks!