Async message delivery and improvements for Subscriptions

[alexander-yevsyukov]

This PR:

• Extends the Delivery class with the factory method for creating local Delivery which handles each message in a separate thread (by @armiol). This is needed for simulating distributed handling of messages (e.g. in example apps). The default behaviour of Delivery is synchronous.
• Transforms Subscriptions class from being a utility into a collection of Subscriptions made by a `Client. This will simplify the management of subscriptions in a client apps.

TODOs added by this PR in the Subscriptions class for improving handling of negative Acks will be addressed shortly in following PRs.

[codecov]

Codecov Report

Merging #1265 into master will decrease coverage by 0.02%.
The diff coverage is 94.20%.

@@             Coverage Diff              @@
##             master    #1265      +/-   ##
============================================
- Coverage     91.06%   91.03%   -0.03%     
- Complexity     4656     4665       +9     
============================================
  Files           598      597       -1     
  Lines         14767    14801      +34     
  Branches        841      843       +2     
============================================
+ Hits          13447    13474      +27     
- Misses         1057     1063       +6     
- Partials        263      264       +1