Changes in libtorrent 1.1 mean that alerts are no longer allowed to be
accessed after the next call to pop_alerts.
> It is safe to call pop_alerts from multiple different threads, as
long as the alerts themselves are not accessed once another thread
calls pop_alerts. Doing this requires manual synchronization between
the popping threads.
The solution is to copy the alert attributes and pass that to the
handlers.
Refs: https://github.com/arvidn/libtorrent/issues/2779#3159
be74d96c6a