Fix DnsTlsSocket fast shutdown path
Previously, DnsTlsSocket's destructor told the loop thread to
perform a clean shutdown by closing an IPC file descriptor.
However, the IPC file descriptor is now an eventfd, which does
not alert the listening thread when it is closed.
This change uses the eventfd counter's sign bit as an indication
that the destructor is requesting an immediate close.
Test: Includes regression test.
(cherry picked from commit 83eccadc7e9d0ee0f75aab980cfdc2159c4c98a2)
3 files changed