ZOOKEEPER-2184: Resolve address only on demand.

[geek101]

Wrap hostname and port into a new ServerCfg class and fix
all the places to use it instead of InetSocketAddress.

This class can be used in the future to encapsulate other
nice config information for example certificate
fingerprint associated with the host etc.

[adyach]

@geek101 will it help to update the list of the ip address in case I use load balancer in connection string? I am asking since I have not found when zookeeper updates list of got ip address. In case we have a situation, when all ip address are not valid anymore.

[adyach]

Seems like this is #150

[geek101]

@adyach can you give me an example of a connection string that you are implying this will help me understand the problem better. Since the client code needs ip addresses of the ZK ensemble, does the DNS resolution of this load balancer hostname supposed to return that set of ip addresses?

[adyach]

your.zookeeper.loadbalancer
The problem is that it is resolved only once when connected, but instances behind load balancer can be replaced.

[geek101]

@adyach this patch's goal is to make sure when ever a new connection is being established DNS lookup will be performed. If you have given a hostname in connection string then if the underlying IP is gone the TCP connection will break and new TCP connection will be attempted by Zookeeper code and at this time DNS lookup will again be performed.

I hope this helps if you can post your example of connection string it will be more helpful. Zookeeper connection string is supposed to contain the ZK ensemble set/subset hostnames/ips.