Ensure NodeJS sample code terminates

Node examples generally don't terminate: in discussion with Sergey Avseyev and Brett Lawson we established that this is due to a bug filed against libcouchbase: Node sdk has

no way of knowing the difference between sockets actively processing a request, and those that are just connected, so unless you manually call cluster.close(), uv thinks there are still things happening and requires Ctrl-C.

While this may disappear if the uv bug is fixed, there's no harm in being explicit with cluster.close (and other SDK examples, for example Go, do this consistently).