Allow using custom Session class

[meetwudi]

Current Session class stores user context in memory. This is legit when we are only serving bot on single machine. Once we are going to serve it onto multiple servers behind load balancer, we should allow user to implement their own Session logic to utilize Redis, Memcache etc.

[jcampbell05]

@tjwudi Yeah we need to implement implementations for Redis.

The Session is a bit of a weird class right now and in theory some of its behaviour could be handled by the Bot or the Client.