Upgrade/downgrade HTTP messages

[krizhanovsky]

Required for #755.

The 2 types of HTTP connection upgrade (explicit support of Upgrade header RFC 7230 6.7 and implicit HTTP/1.0 to/from HTTP/1.1) must be implemented.

Implicit HTTP/1.0 <-> HTTP/1.1 conversion

Tempesta must upgrade HTTP/1.0 requests to HTTP/1.1 on forwarding them to upstream server (if ther server supports HTTP/1.1) and vise versa downgrade the server responses on forwarding them to a client. Web-cache stored responses must be upgraded/downgraded to appropriate HTTP version.

See Advertise HTTP/1.1 Correctly.

Also RFC 7230 4.1.2 discusses de-chunking of messages sent to HTTP/1.0 clients, which also must be implemented.

The Internet adopts HTTP/3 nowadays, so there is no sense to care about HTTP/1.0.

Upgrade header

Upgrade header RFC 7230 6.7 must be implemented for Websocket (#755) protocol. Note that we do not support h2c, so Upgrade: h2c should be just ignored.

Tests

Please develop corresponding functional test in separate PR.

[krizhanovsky]

We need Upgrade for websockets #755 only, so there is nothing to do with a separate issue.