Replace the HTTP header maps with glz::http_headers

[annihilatorq]

Ports the net stack to glz::http_headers (#2709).

Rewriting the header lookups for the new container resulted in fixing two WebSocket bugs. glz::http_headers keeps repeated field names instead of overwriting them, and its .contains_token() replaced the hand-rolled substring and prefix comparisons, so every lookup had to be revisited, and both were found there.

WebSocket implementation bugs

1 - the handshake response check never compared a full field name. The code used a comparison like this:
strncasecmp(name.c_str(), "Upgrade", 7) for the headers "Upgrade", "Connection" and "Sec-WebSocket-Accept" comparing the first N bytes using strncasecmp, where N is the length of the expected header name. The problem is that this approach would accept headers such as "Connection-Something" as valid because the name starts with "Connection", and so on with other headers listed previously.

Example - the old client accepted this response even though the server never sent an Upgrade field:

HTTP/1.1 101 Switching Protocols\r\n
Upgrade-Extra: websocket\r\n
Connection: Upgrade\r\n
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=\r\n\r\n

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2 - the client is allowed to send a list-based field not as a comma-separated list, but as separate header fields separated by CRLF. The previous issue was that std::unordered_map used a "last-wins" approach, thereby discarding one of the values for that header. In such cases, user could only hope that the last header received contained the required token ("Connection: Upgrade") as the header-field value.

Example - the old server refused this valid request because keep-alive arrived last:

GET /ws HTTP/1.1\r\n
Host: 127.0.0.1:8080\r\n
Upgrade: websocket\r\n
Connection: Upgrade\r\n
Connection: keep-alive\r\n
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==\r\n
Sec-WebSocket-Version: 13\r\n\r\n