HTTP/2 follow-up

[alexeyr-ci]

• Upgrade HTTPX to 1.4.3 when released (1.4.2 has a bug)
  • Replace max_concurrent_requests by the new option pool_options.max_connections_per_origin. See If my program connects only to a single origin, is there a difference between `max_concurrent_requests` and `pool_options.max_connections_per_origin`? HoneyryderChuck/httpx#68, it seems to be a better fit.
• Try to check if requests are actually multiplexed over a single connection, since we aren't sending them in a single request call (see the same discussion). For the test, add some sleep in workers and open the same dummy page in separate tabs/browsers. Would be even better to actually do it from separate computers...
• If they aren't, consider contributing to HTTPX (or switching to yet another library, but I'd prefer to avoid it).
• In particular, "When the HTTP/2 TCP connection is established, the client and server exchange SETTINGS frames first, which indicates how many streams can be open at a time(or how many parallel requests)". What's in our SETTINGS by default? Should we add a configuration option? EDIT: 100 by default, that should be fine.