Add support to mapping between internal and external IP's and Validate fileds from 'SENTINEL sentinels' command

[erezleaks]

In case sentinels are communicating with each other with internal IP there is a need to add host mapping to external IP so when sentinel returns the IP of redis master it will be accessible for hosts out side.
Usage example:

var sentinel = new RedisSentinelManager(new string[]{ "10.0.0.1:26379", "10.0.0.2:26379", "10.0.0.3:26379"});
sentinel.AddHostMapping(new System.Collections.Generic.Dictionary<string, string> { { "10.0.0.1", "35.36.37.38" }, { "10.0.0.2", "35.36.37.39" }, { "10.0.0.3", "35.36.37.40" } });

In addition, fix renamed or moved fields in 'SENTINEL sentinel' command.
for example see "pending-commands" => "link-pending-commands":
https://groups.google.com/forum/#!topic/redis-db/24GIAEvW9VU