Find a way to prevent people from writing synchronous C# code

We ran into a couple cases (one [here](https://social.msdn.microsoft.com/Forums/azure/en-US/5c142d90-adef-4d7b-8a30-3224b6229d93/app-could-be-having-an-intermediat-issue-or-failing-to-start?forum=AzureFunctions)) where user's apps deadlocked because they were making synchronous outgoing http calls, e.g.

```
HttpResponseMessage response = client.PostAsync(url , httpContent).Result;
```

That causes the thread to be blocked while waiting for the response. Once you throw enough traffic at it, all the threads are blocked, and there are none left to get scheduled and do the work, so you have a deadlock. See http://stackoverflow.com/questions/10343632/httpclient-getasync-never-returns-when-using-await-async.

@lindydonna suggested that we could potentially use Roslyn's analyzer to detect the situation and treat it as an error. Though it's an aggressive move, we really want to make it very hard for people to do this.

Start by reviewing the proposed Roslyn analyzer approach and the synchronous HttpClient example, `client.PostAsync(url, httpContent).Result`. Determine how the analyzer should identify synchronous outgoing HTTP calls and whether treating them as errors is feasible. Done means the agreed cases are detected reliably and covered by analyzer tests.

Written by the indexing model from the issue text.