Tap on another ring and gets its status

[aravindvs]

@jwolski - as we discussed this is the tapring patch (pull model)..

This patch adds a TapRing() API to ringpop which queries the given host to get the status of it's ring dynamically (this is a pull based model rather than a push mode and should generate n/w traffic only on demand).
This will be very useful for front end client applications which doesn't want to participate on the gossip protocol and gives them the ability to get the status of the other ring to make placement/forwarding decisions.

There is also an example usage of this new API:
To use this example:
1. start 4 different instances of this example.
* We have 3 nodes which bootstrap to the ring (by default the bootstrap will happen with these 3 hosts)
go run examples/tapring/main.go --hostport "127.0.0.1:3000"
go run examples/tapring/main.go --hostport "127.0.0.1:3001"
go run examples/tapring/main.go --hostport "127.0.0.1:3002"

   * start an instance which just attaches/taps to the ring
   go run examples/tapring/main.go -attach=true -hostport="127.0.0.1:3003"

2. Use tcurl to lookup servers for the given key by tapping on a host
   tcurl -p "127.0.0.1:3003" listenring "/lookup" --raw -3 '{"key":"hello","taphost":"127.0.0.1:3000","replicas":3}'
 As and when you kill the original nodes, lookup will give you different results.

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✅ dansimau
✅ motiejus
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❌ CorgiMan
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