Refactored Subscribe Handler

[gnought]

No description provided.

[mcollina]

Where does this restore property come from? Maybe we could avoid setting it in the first place.

[gnought]

I think it is best to use connecting flag #409 to replace restore property where defined in https://github.com/moscajs/aedes/blob/master/lib/handlers/connect.js#L179

[gnought]

Users may hook broker client event to do a subscribe. The connecting is true during client event emits. In this case users cannot subscribe a topic if we use connecting to replace restore property. As we support queueing messages before CONNECT, I abandon this approach.

Previous restore is added as an item in packet object in handleSubscribe to handle restored subscription behaviour (https://github.com/moscajs/aedes/blob/master/lib/handlers/connect.js#L181), it is not safe if users inject this key. My approach is to remove it, and add one more arguments in handleSubscribe.

In addition handleSubscribe is refactored to handle normal and restored subscription separately. Simpler and readable.

[gnought]

@mcollina does this PR look goods to you?

[mcollina]

LGTM