Implement support for suspend/resume

[cgillum]

Fixes #181

The MSSQL backend was missing support for suspend/resume. It also made some bad assumptions about how it would eventually behave when introduced, so some existing logic needed to be removed.

I also bundled a couple additional minor fixes into this PR:

• Fixed inconsistent exception throwing in SqlOrchestrationService.WaitForOrchestrationAsync (realized this was a problem while writing my suspend/resume test)
• Fixed minor issue with default DateTime handling (force it to always be UTC) - this is an issue with certain kinds of serializers, like protobuf, which reject DateTime values that aren't explicitly UTC (found this doing some unrelated exploratory prototyping).

[cgillum]

Adding a few comments with additional context.

[cgillum]

This was added long before we actually implemented suspend/resume in DurableTask.Core, and was based on the assumption that we wouldn't even load suspended orchestrations. In the final design, however, we decided that we needed to continue loading suspended orchestrations, so this line needed to be removed.

[cgillum]

The above code was deleted to ensure that we only throw OperationCanceledException (or TaskCanceledException) if this times out. Otherwise, sometimes we'd throw TimeoutException and sometimes we'd throw OperationCanceledException, depending on which line of code was running when the cancellation token was signaled.

[cgillum]

Needed to add this, otherwise we can't write test code that waits for timeouts (like I do in my new test) when a debugger is attached.

[bachuv]

LGTM!