Add convenience method to get session TTL

[theetrain]

Hello, thanks for this utility.

I'm using sessionManager to store a cached copy of a session managed by a separate service. It would be helpful to have convenience methods to compare it with the session used by my external service, including:

• sessionManager.ttl() - get the remaining ttl of the current session

My current workaround is to reuse the redis client to fetch the TTL manually from the cookie value.

[theetherGit]

@theetrain Ok, I'll think about it. Currently I have a full time job and not getting time to fix things. If you can provide me with a base implementation then I'll review an merge it.