Add an /n_status Message and equivalent messages

rfcs/0013-Add-n_status-Message.md

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+- Title: Add an /n_status Message and equivalent messages
+- Date proposed: 2021-01-12
+- RFC PR: https://github.com/supercollider/rfcs/pull/0013
+
+# Summary
+
+Add an /n_status message to the OSC command interface, to allow you to check on whether a node currently exists and if it is running or paused.
+
+# Motivation
+
+Given the complications of asynchronous communication, it can be useful to check whether a node already or still exists. (A recent case arose in the VST interface). Currently there is no straightforward way to do this. One can send an /n_query but if the node does not exist this will cause a FAILURE message to be posted (in this case the node not existing is not a failure, just info that we need). This seems like a basic thing that we should be able to query. It would also be useful in multi-client situations, where another client may have created a node.
+
+The existing NodeWatcher approach does not suffice here, since there is no way to reliably register a node that may not exists. Enabling this would require such a message.
+
+As we're doing this we may as well include info on whether the node is running or paused. Anything else?
+
+# Specification
+
+/n_status - Check whether a node exists and its run status.
+
+N * int	node ID
+The server sends an /n_status.reply message for each node to registered clients.
+
+/n_status.reply has the following arguments:
+
+int node ID
+int status flag, 1 if running, 0 if paused, -1 if node was not found
+
+In addition we will add a corresponding message to the Node class, and optionally a checkNodeStatus method to the Server class. (Thoughts on this welcome!) Other places in the class lib?
+
+# Drawbacks
+
+Adds another command to the interface.
+
+# Unresolved Questions
+
+Whether to add a method to Server. My instinct is no, as it's a bit odd style, and Server is already huge.