abstract_tcp_server2: fix timeout on exit

[moneromooo-monero]

When closing connections due to exiting, the IO service is
already gone, so the data exchange needed for a gracious SSL
shutdown cannot happen. We just close the socket in that case.

[fluffypony]

Reviewed

[vtnerd]

Can you describe how this was tested? I would like to do some testing myself, particularly with ASAN/MSAN, unless someone has already done that.When the io_service is destroyed, all pending async operations should be called with the aborted error. Somehow connection objects are still hanging around somewhere?

And this fix should be racy - a lock should be required here. Even an atomic flag won't guarantee correct ordering.