keepalive connections cache between 2 proxy

[foxgab]

hello,

my app will keep a long connection with server, while server may push
message to client, two levels nginx proxy for the upstream servers, does
keepalive directive support this model?

the documentation of the keepalive directive said: 
"For HTTP, the proxy_http_version directive should be set to '1.1' and the
'Connection' header field should be cleared."

why clear that header?

if client or server close the connection, will the connection between two
nginx still keepalive?

will nginx creat connection with upstream servers even if there's no request
received?
will nginx pass requests from different clients to upstream in one
keep-alive connection?

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,275466,275466#msg-275466