websocket: Add warning if client connection isn't closed cleanly

websocket: Add warning if client connection isn't closed cleanly

This gives a warning that is not dependent on GC for the issue
in #3257. This new warning covers all websocket client connections,
while the previous GC-dependent warning only affected those with
ping_interval set. This unfortunately introduces an effective
requirement to close all websocket clients explicitly for those
who are strict about warnings.