FIX: hub callbacks registration and their IDs related to protocol 1.2

### Synopsis of problem:
SRHub invoke never get response to completion block due to incorrect use of callback registration and response handling.

### Fixed:
 1. SRHub callback incorrectly registered:
   *  _callbacks dictionary was not initialized in all constructors
   *  _callbackId stored as a key and then incremented and used in request so never found back in _callbacks dictionary when comparing with response (response is bigger by one).
 2.  _SRHubConnection_ `didReceiveData:` method not called in response on "invoke" request to handle SRHub registered callback.
 3. Minor change: set property attribute to weak for object property in SRSubscription for auto-zeroing so be able to see when object is already released