transport: cache default ssl context

[karolinepauls]

Summary

Creating a client takes 25ms of CPU time. This is a lot. Most of that time is spent initialising the SSL context. Since the context is private anyway, we can cache it. In order to avoid memory problems and privacy issues, only the default context is cached.

Prior discussion, when the problem was left for later (2022): #2298 (comment). Personally, I'm finding solving this problem in httpx to be a comparable amount of work to solving it in applications.

Before:

In [2]: timeit httpx.AsyncClient()
26.6 ms ± 364 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)

In [3]: timeit httpx.Client()
26.8 ms ± 268 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)

After:

In [2]: timeit httpx.AsyncClient()
46.2 µs ± 224 ns per loop (mean ± std. dev. of 7 runs, 10,000 loops each)

In [3]: timeit httpx.Client()
46.5 µs ± 1.14 µs per loop (mean ± std. dev. of 7 runs, 10,000 loops each)

CPU Profile (only before):

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.