Switch Async HTTP-client to `httpx` (HTTP/2-support)

[thomasht86]

I confirm that this contribution is made under the terms of the license found in the root directory of this repository's source tree and that I have the authority necessary to make this contribution on behalf of its copyright owner.

• [x] Removed @staticmethod-decorator of callback_docv1 (See comment on bug below)
• [x] Switched to httpx - Mainly to add HTTP/2 support, which presumably could improve feed performance by a lot.
• [x] Unit tests passing ✅ - BUT, working to quantify performance gain.

⚠️ Encountered this bug when tenacity retry is applied

```python self = , wait=, before=, after=)> retry_state = def iter(self, retry_state: "RetryCallState") -> t.Union[DoAttempt, DoSleep, t.Any]: # noqa fut = retry_state.outcome if fut is None: if self.before is not None: self.before(retry_state) return DoAttempt() is_explicit_retry = fut.failed and isinstance(fut.exception(), TryAgain) if not (is_explicit_retry or self.retry(retry_state)): return fut.result() if self.after is not None: self.after(retry_state) self.statistics["delay_since_first_attempt"] = retry_state.seconds_since_start if self.stop(retry_state): if self.retry_error_callback: > return self.retry_error_callback(retry_state) E TypeError: 'staticmethod' object is not callable notebooks/lib/python3.9/site-packages/tenacity/__init__.py:322: TypeError ```

We should make unit tests for error handling. Maybe this is part of your fix?

[jobergum]

Last time I tried httpx with http/2 I ran into issues with that it did not handle the case when the server would shutdown the connection (we do in Vespa cloud to disallow extremely long connections)

[thomasht86]

Last time I tried httpx with http/2 I ran into issues with that it did not handle the case when the server would shutdown the connection (we do in Vespa cloud to disallow extremely long connections)

That's very useful info. I will try to mock a test for that to check!

[thomasht86]

Closing for now, as httpx did not improve feeding performance notably, even with HTTP/2. Need to investigate further to justify the change.