Don't leak the viewport subscription when the viewport data is read

[niloc132]

Fixes #4410

[mattrunyon]

Looks like it's working properly in the docs snapshots

[niloc132]

This is on hold to address a few other leaks as well.

[mattrunyon]

Should this close #4410?

Tested w/ docs snapshot tool by undoing the mitigations I had put in.

Confirmed w/ latest docker image the number of listeners grows unbounded by creating my own class extending ws which increments/decrements on newListener and removeListener.

With this change, number of listeners was not growing and stayed at a consistent overhead of 12 even after closing/re-opening the connection.

[niloc132]

Fixed the linked issue, thanks.