`curl_easy_recv` must be called in a loop until it returns EAGAIN,
because it may cache data, but only activate the read notifier once.
Additionally, the data received can contain multiple WebSocket frames
and only activate the notifier once, so we have to keep reading frames
until there isn't enough data.
We also have to do this immediately after connecting a WebSocket,
since the server may immediately send data when the WebSocket opens
and before we create the read notifier.
This makes Discord login faster and more reliable, and makes Discord
activities start loading.
The assertion in `WebSocketImplCurl::did_connect()` keeps failing for
multiple websockets when loading `https://www.speedtest.net/` since
commit 14ebcd4. This fixes that by checking and returning false if
something went wrong and letting the caller function handle it.
This implementation can be better improved in the future by ripping
out a lot of the manual logic in LibWebSocket and rely on libcurl to
parse our message payloads. But for now, this uses the 'raw mode' of
curl websockets in connect-only mode to allow for somewhat seamless
integration into our event loop.
