This PR is derived from PR-692 and have been merged with the latest master commit.
Things that have been done:
1. review code for PR-692
2. fixed potential infinite loop in the code
3. Adapt code to success build with the latest master commit
4. Manual testing on 3 Sentinel nodes and 3 Redis nodes (connection and failover)

Usage:
```C#
                ConfigurationOptions sentinelConfig = new ConfigurationOptions();
                sentinelConfig.ServiceName = "mymaster";
                sentinelConfig.EndPoints.Add("192.168.99.102", 26379);
                sentinelConfig.EndPoints.Add("192.168.99.102", 26380);
                sentinelConfig.EndPoints.Add("192.168.99.102", 26381);
                sentinelConfig.TieBreaker = "";
                sentinelConfig.DefaultVersion = new Version(4, 0, 11);                 
                // its important to set the Sentinel commands supported
                sentinelConfig.CommandMap = CommandMap.Sentinel;

                // Get sentinel connection
                ConnectionMultiplexer sentinelConnection = ConnectionMultiplexer.Connect(sentinelConfig, Console.Out);
                // Create master service configuration
                ConfigurationOptions masterConfig = new ConfigurationOptions { ServiceName = "mymaster" };
                // Get master Redis connection
                var redisMasterConnection = sentinelConnection.GetSentinelMasterConnection(masterConfig);

                ...
               IDatabase db = redisMasterConnection.GetDatabase();                
               db.StringSet(key, value);
               ...
               string value1 = db.StringGet(key);

```