When a connection is closed, several events can be involved (and can be triggered).
Some of these events will attempt to close the connection (if it hadn't been already).
This, at best, least to multiple invocations of close-listeners. At worst, a loop of
close-calls is created.

This commit replaces the two-way boolean that guards closure (isClosed) with a
three-way guard. Now, a distinction is made between between a connection that is
closed, and one that is closing.

Additionally, some null pointer guards have been added, as I've seen those pop up in
my local logs during development.