os-nginx: Add option for upstream keepalive

[sigurdvaa]

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Is your feature request related to a problem? Please describe.
For performance tuning I would like to be able to enable keepalive for upstream connections.

Describe the solution you'd like
Two new options in Upstream:

• Keepalive: how many connections to keep alive upstream (0 or empty for disabled)
• Keepalive Timeout: how long to wait for closing an idle upstream connection

Describe alternatives you've considered
Not really any alternatives, as the keepalive directive must be set in the upstream context, which will be overwritten when pressing Apply in the WebUI.

Additional context
This will require two new directives in the upstream context and two in the location context when upstream keepalive is enabled:
upstream

• keepalive [int];
• keepalive_timeout [int];

location

• proxy_http_version 1.1;
• proxy_set_header Connection "";

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[1249993110]

I also need proxy_http_version.