Convert sydent/http/srvresolver.py and associated files to async/await

[H-Shay]

This PR is the result of converting the file sydent/http/srvresolver.py to async/await, and updating the return value in other files that use or call the converted method (there was only one method in srvresolver.py which was converted.

[H-Shay]

Ahh the 3.6 unit tests are failing (I suspect) because the older versions of mock don't automatically convert an asynchronous function to an asyncMock when patched in. I'll figure it out monday morning!

[clokep]

because the older versions of mock don't automatically convert an asynchronous function to an asyncMock when patched in. I'll figure it out monday morning!

We define an async mock somewhere in the Synapse test suite. (There's a simple_async_mock function.)

[H-Shay]

So-after much researching and reading, (I haven't used mocks before, it's new to me!) I found a solution, which is to create an AsyncMock class and use it in the patch. I looked in synapse and found the function you were talking about, although I couldn't figure out how to port that definition to this situation, as I think I needed a class (to create an object) rather than a function. So I went with some code I found on Stack Overflow, and rather than defining the AsyncMock class in the test_blacklisting.py file, I stuck it in test.utils. I am sure this is not the most graceful way to handle all this, but it seems to be working.

[clokep]

Looks great, just a couple of minor comments!