Upstream: least_time balancer module

Upstream: least_time balancer module

The module implements load-balancing algorithm based on least average
response header/last_byte time and least number of active connections.

The optional "inflight" mode enables accounting of incomplete
requests/sessions.  This allows to mitigate cases when an upstream
server hangs and does not close connections.

Example configuration:

upstream u {
    least_time header | last_byte [inflight];
    server a;
    server b;
}

Co-authored-by: Roman Arutyunyan <arut@nginx.com>