Fix a race condition of not getting all the output in .NET Image

[mikhailshilkov]

Fix #200

There is a race condition in the .NET SDK. As observed in a local test, Exited event may fire before the last OutputDataReceived call. If that happens, the last chunk of the output won't be appended to stdout. The inspect command returns a single chunk of output. If it's missing, the output is empty, and the exception "No digest available for image" is fired.

One possible fix would be to wait for a call with e.Data equal to null which signals the completion of the output.

A simpler solution is suggested here and implemented in this PR: make and extra WaitForExit call in the ProcessExited handler. This flushes the buffers and lets the output sync in. My local tests confirmed the improvement: I don't get the issue anymore.

Repro program that I used: https://gist.github.com/mikhailshilkov/978ae50e26c6e712c56fbd4d73b48222

In addition, I moved Exited event handler registration to before the Start() call to avoid a potential miss. (I believe this is not related to this issue, though.)

cc @Aaronontheweb @dbeattie71

[justinvp]

LGTM