Added a check for DNS updates

[barshaul]

Before reusing an old connection, check if its node has encountered a DNS change, where the node's endpoint now leads to a different IP address.

Implementation detailes:
connect_simple now returns both the connection and an optional String with the connection IP (only valid for TCP connections, for Unix returns None).
The cluster client now holds a ConnectionMap from the node's name to ClusterNode entries that holds both the connection and the IP. When we check if existing connection is valid in the get_or_create_conn function, we now also check if the node's address still points to the same IP.

[nihohit]

please rebase over https://github.com/nihohit/redis-rs/pull/29 instead of the bad lint fix.

[nihohit]

If this change is only for the cluster's sake, let's save the IP in the cluster instead of adding it to each connection.
Adding get_ip to the ConnectionLike trait makes this a breaking change.

[barshaul]

@nihohit Ready for another review

[nihohit]

did you benchmark this and verify there's no perf degradation?

[nihohit]

FSM, all comments are optional.

Please benchmark before merging, to verify there's no perf harm. Is it possible to test this with Polyglot before merging, to see if it changes the behavior there?

[barshaul]

@nihohit ready for a review with the new connections container