upstream prematurely closes cnx => 502 Bad Gateway to client

[stefws]

@B.R. You're right, seemed my upstream tomcat instances were RESETing cnx as
reply something.

So far I improved it a lot my altering a http connector keepAliveTimeout
value from mistakenly expressed as sec when in fact it should be msec ;)

When heavy load it still occurs but far less frequently, will dig deeper
into tomcat trimming/tunning

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,265031,265070#msg-265070