Initial pass at adding Sentinel support. Adds ConnectionMultiplexer that
is managed by a set of Sentinel servers. Usage:
```C#
ConfigurationOptions sentinelConfig = new ConfigurationOptions();
sentinelConfig.ServiceName = "ServiceNameValue";
sentinelConfig.EndPoints.Add("127.0.0.1", 26379);
sentinelConfig.EndPoints.Add("127.0.0.2", 26379);
sentinelConfig.EndPoints.Add("127.0.0.4", 26379);
sentinelConfig.TieBreaker = "";
sentinelConfig.DefaultVersion = new Version(3, 0);

// Add the configuration for the masters that we will connect to
ConfigurationOptions masterConfig = new ConfigurationOptions();
sentinelConfig.SentinelMasterConfignfigurationOptions = mo;

// Connect!
ConnectionMultiplexer sentinelConnection =
ConnectionMultiplexer.Connect(sentinelConfig);

// When we need to perform a command against the masters, use the master
connection
sentinelConnection.SentinelMasterConnection.GetDatabase().MyCommand();
```

Sentinel Support - Second Draft

Modified approach to allow for a sentinel-only setup with the option of
requesting a connection to a master managed by the sentinel connection.
Can also support multiple services on the same sentinel.

Handle closed managed connections

Add some basic handling for disposed connections that are managed by the
Sentinel connection.