Extension of Redis Session Store to Support Redis Cluster (#363)

* Extend the redis session store to support redis cluster

* rename function newRedisClient to newRedisCmdable

* update docs about redis cluster as session store

* update autocomplete script with redis cluster options

* add check about conflict between option redis-use-sentinel and redis-use-cluster

* update change log

* Update docs/configuration/sessions.md

Co-Authored-By: Joel Speed <Joel.speed@hotmail.co.uk>

* Update pkg/sessions/redis/redis_store.go

Co-Authored-By: Joel Speed <Joel.speed@hotmail.co.uk>

* add the dropped option back

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>