Future state machine rewrite. (#279)

* Future state machine rewrite.

The polling mechanism for async operations is determined by the verb of
the originating request along with the HTTP status code of the initial
response; the original implementation didn't take this into account
which caused some Future types to incorrectly poll or obtain the wrong
URL for retrieving the result.
This change introduces the pollingTracker interface as an abstraction
over verb-specific polling trackers; this allows the logic to be
separated per tracker.  All shared state and methods are in the
pollingTrackerBase type.

* Added method WaitForCompletionRef to avoid breaking change.

* Updated CHANGELOG and renamed method.

* Reformat deprecated methods list.

CHANGELOG.md

@@ -1,5 +1,23 @@
 # CHANGELOG
 
+## v10.9.0
+
+### Deprecated Methods
+
+| Old Method | New Method |
+|-------------:|:-----------:|
+|azure.NewFuture() | azure.NewFutureFromResponse()|
+|Future.WaitForCompletion() | Future.WaitForCompletionRef()|
+
+### New Features
+
+- Added azure.NewFutureFromResponse() for creating a Future from the initial response from an async operation.
+- Added Future.GetResult() for making the final GET call to retrieve the result from an async operation.
+
+### Bug Fixes
+
+- Some futures failed to return their results, this should now be fixed.
+
 ## v10.8.2
 
 ### Bug Fixes