Before changing _endpoint_state_map to hold a
lw_shared_ptr<endpoint_state>, provide synchronous helpers
for users to traverse all endpoint_states with no need
to copy them (as long as the called func does not yield).
With that, gossiper::get_endpoint_states() can be made private.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
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