Add the Client.install_asyncio_hook function.

[Kentzo]

asyncio needs special handling as its unhandled exceptions are consumed by logging and ignoring them.

@asvetlov Please take a look at the implementation and let me know if handling can be improved. I used implementation of BaseEventLoop.default_exception_handler as a reference but didn't check all available context's keywords in all released versions of Python.

[Kentzo]

I'm not sure how to cause asyncio to call exception handler without 'exception' but with handle of future with the _source_traceback attribute set.

[Kentzo]

@ashwoods I used the 'exception' level instead of 'fatal' as handled errors don't explicitly cause interpreter to exit by default. Was it a good idea?

[Kentzo]

@ashwoods Is there anything I should change / fix?

[asvetlov]

The idea is strange for me.
Why is it a part of raven-python?
I thought raven-aiohttp is better place for asyncio-related things.

[Kentzo]

@asvetlov This change is to handle asyncio exceptions and unrelated to aiohttp transport.

In one of the projects we are using this without raven-aiohttp.

[Kentzo]

@ashwoods I would love to address the issues if there are any

[Kentzo]

Anyone? I'm willing to fix any issues.

[ashwoods]

I haven't had time to look into context in the loop. Will breadcrumbs leak and be cleared?

[Kentzo]

@ashwoods What test would show it?

[Kentzo]

Re-opened to re-run CI tests.

@ashwoods Any thoughts?