reverseproxy: Fix overwriting of max_idle_conns_per_host (closes #4201)

Also split the Caddyfile subdirective keepalive_idle_conns into two properties so the conns and conns_per_host can be set separately.

This is technically a breaking change, but probably anyone who this breaks already had a broken config anyway, and silently fixing it won't help them fix their configs.

modules/caddyhttp/reverseproxy/reverseproxy.go

@@ -204,7 +204,7 @@ func (h *Handler) Provision(ctx caddy.Context) error {
 			KeepAlive: &KeepAlive{
 				ProbeInterval:       caddy.Duration(30 * time.Second),
 				IdleConnTimeout:     caddy.Duration(2 * time.Minute),
-				MaxIdleConnsPerHost: 32,
+				MaxIdleConnsPerHost: 32, // seems about optimal, see #2805
 			},
 			DialTimeout: caddy.Duration(10 * time.Second),
 		}