Add simple Redis SessionRepository implementation

[vpavic]

This is a PR to address #1278.

[rwinch]

@vpavic Can you add a sample that uses this?

[vpavic]

Some things to point out about this implementation, since they differ from RedisOperationsSessionRepository but I believe are the way to go in future:

• the #setKeyNamespace is transparent in this implementation whereas RedisOperationsSessionRepository appends :, which both adds complexity and is obviously not so transparent
• it uses RedisOperations<String, Object> vs RedisOperations<Object, Object> in RedisOperationsSessionRepository (FWIW ReactiveRedisOperationsSessionRepository uses ReactiveRedisOperations<String, Object>)
• it aligns with Reconsider handling of SessionRepository#save for invalidated sessions #1277 by throwing IllegalStateException if #save is attempted for an invalid (i.e. not present in data store) session

The following open points are:

• naming: something like DefaultRedisSessionRepository or BasicRedisSessionRepository comes to mind - we can also rename RedisOperationsSessionRepository to something more appropriate
• configuration support: IMO this doesn't need to be imminent for 2.2

Note that I've also added some common Redis support related code in the first commit in this PR.

[vpavic]

@vpavic Can you add a sample that uses this?

Yes, that would be a good thing to have.

[vpavic]

I've added a sample application to showcase the new implementation.