Currently snitch explicitly calls storage service (if
it's initialized) to update topology on snitch data
change.
Instead of it -- make storage service subscribe on the
snitch reconfigure signal upon creation.
This finally makes snitch fully independent from storage
service.
In tests the snitch instance is not created, so check
for it before subscribing.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
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