c$conn is often needed for connection events, but it being established
in connection_state_removed can be a problem because event handlers have
to remember to call Con::set_conn().
This commit moves to call Conn::set_conn() in new_connection.
Addresses GH-4202
update logs
This wasn't possible before #3028 was fixed, but now it's safe to set
the value in new_connection() and allow other users access to the
field much earlier. We do not have to deal with connection_flipped()
because the community-id hash is symmetric.
Issue #3028 tracks how a flipped connections reset a connection's value
including any state set during new_connection(). For the time being,
update community-id functionality back to the original connection_state_remove()
approach to avoid missing community_ids on flipped connections.
