Server PUSH

[krizhanovsky]

HTTP/2 server PUSH for proxy and server move (if #471 (Web-server mode) is done by the time of the issue) should be implemented. Link header (RFC 5988) with preload must be used for proxy mode. See corresponding Nginx http2_push_preload option and how H2O handles the header, e.g. CASPer. Another option, which must be implemented is 103 Early Hints (see Reorganizing Website Architecture for HTTP/2 and Beyond slides 38-39).

Server pushed responses must be cached according to RFC 7540 10.4 with the necessary server authority validation. When a cached response is pushed, the h2 priority must be determined using the priority attribute of the MIME-type configuration (see H2O for an example), so a MIME response parser as for #902 is required.

Pushed responses must set header x-http2-push: pushed.

In full HTTP/2 proxy mode (#1125) we need just to proxy the server PUSH to a client. PUSH_PROMISE logic must be implemented to initiate PUSH from the cache and stop it if a client responds with RST_STREAM.

We also should implement PUSHin diary as Apache HTTPD.

Cache digests are still in draft, but being accepted must be implemented as part of the issue.

UPD HTTP/2 Server PUSH is going to die. QUIC still requires it though, so the task depends on #724: we need to implement it only if QUIC requires it.