ec2 multi-region snitch: Register helper in .start()
ec2 multi-region snitch: Register helper in .start()

This driver registers reconnectable helper in it gossiper_starting()
callback. It can be done earlier -- in the snitch .start() one, as
gossiper doesn't notify listeners until its started for real (event
its shardow round doesn't kick them).

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla