HTTP/2 should be off by default. #582

So...

I'm super excited that we've got an HTTP/2 implementation in here, but we also need to consider what default behavior we should be aiming for, and what we should prioritise.

Our HTTP/2 implementation is neccessarily going to be more complex, and less mature than our HTTP/1.1 implementation. Even if that wasn't the case, then HTTP/2 infrastructure that we're connecting too is more likely to have implementation issue and quirks than HTTP/1.1 versions.

I think we ought to clearly be prioritizing for reliablity, while still supporting users that want the more cutting-edge functionality.

I'd suggest that:

• We ought to have a documentation page on HTTP/2, with a brief overview of the primary differences, and motivation as to why user might or might not want to switch HTTP/2 support on.
• We ought to tidy up our interface for switching between HTTP versions.

We currently have this...

client = Client(http_versions=["HTTP/1.1", "HTTP/2"])  # Really clunky
client = Client(http_versions=["HTTP/1.1"]) 
client = Client(http_versions=["HTTP/2"])  # Doesn't really make sense - you *might* still end up with an HTTP/1.1 connection.

I think we should just switch to...

client = Client(http_2=True)
client = Client(http_2=False)

Furthermore, I think the default ought to be false. There's no point in exposing our users to this class of issues https://github.com/encode/httpx/issues?q=is%3Aissue+is%3Aopen+label%3Ahttp%2F2 as our default behaviour, but we can still make it easy for our users to upgrade to the HTTP/2 implementation.