When shutdown gossiper may have some messages being processed in
the background. This brings two problems.
First, the gossiper itself is about to disappear soon and messages
might step on the freed instance (however, this one is not real now,
gossiper is not freed for real, just ::stop() is called).
Second, messages processing may notify other subsystems which, in
turn, do not expect this after gossiper is shutdown.
The common solution to this is to run background code through a gate
that gets closed at some point, the ::shutdown() in gossiper case.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
9fccf7f3af