core: shutdown more aggressively in the engine destructor (#540)
core: shutdown more aggressively in the engine destructor (#540)

Description: At the point where this destructor is being called, the OS is already trying to terminate the thread holding Envoy's static state. shutdown() may simply not be aggressive enough to tear things down in time. This will cause the event loop to exit.

Signed-off-by: Mike Schore <270e364e708f2af319f244c09ceba52a03dbd980@gmail.com>
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