shutDownGracefully
Initiates a graceful shutdown of the web server. Handling of new requests is prevented and the given callback
is invoked at the end of the attempt. The attempt can be explicitly ended by invoking stop. The default implementation invokes the callback immediately with IMMEDIATE, i.e. no attempt is made at a graceful shutdown.
Since
2.3.0
Parameters
callback
the callback to invoke when the graceful shutdown completes