Add support for a high availability redis

[bennsimon]

At the moment openSRP only supports redis standalone setup which does not offer high availability or scaling features. To increase the availability of redis we need to add support for redis sentinel or redis cluster since they both support automatic failover incase one of its nodes goes down. However on top of automatic failover redis cluster offers scaling with data sharding which would be suitable for handling larger data and providing faster access to it.

[bennsimon]

Done