Telnet server accumulated dead connections

Because:
* There is no builtin to get a list of open connections, and
* There is no builtin to test to see if a given connection is still open, and
* There are certain circumstances under which the `.onClose` method of a connection object may not be successfully run to completion (such as when the connection is closed by virtue of the server being shut down)

there is a tendency for `$.servers.telnet.connections` to fill up with a lot of cruft, which in turn shows up on the `$.console` display.

Some effort was made previously to mitigate this, but it has evidently not been sufficient. See if it's possible to make this work well-enough in-database, or open a new bug to provide the necessary builtins.