reverseproxy: Adjust defaults, document defaults (#4436)

* reverseproxy: Adjust defaults, document defaults

Related to some of the issues in https://github.com/caddyserver/caddy/issues/4245, a complaint about the proxy transport defaults not being properly documented in https://caddy.community/t/default-values-for-directives/14254/6.

- Dug into the stdlib to find the actual defaults for some of the timeouts and buffer limits, documenting them in godoc so the JSON docs get them next release.

- Moved the keep-alive and dial-timeout defaults from `reverseproxy.go` to `httptransport.go`. It doesn't make sense to set defaults in the proxy, because then any time the transport is configured with non-defaults, the keep-alive and dial-timeout defaults are lost!

- Sped up the dial timeout from 10s to 3s, in practice it rarely makes sense to wait a whole 10s for dialing. A shorter timeout helps a lot with the load balancer retries, so using something lower helps with user experience.

* reverseproxy: Make keepalive interval configurable via Caddyfile

* fastcgi: DialTimeout default for fastcgi transport too

modules/caddyhttp/reverseproxy/reverseproxy.go

@@ -204,14 +204,7 @@ func (h *Handler) Provision(ctx caddy.Context) error {
 
 	// set up transport
 	if h.Transport == nil {
-		t := &HTTPTransport{
-			KeepAlive: &KeepAlive{
-				ProbeInterval:       caddy.Duration(30 * time.Second),
-				IdleConnTimeout:     caddy.Duration(2 * time.Minute),
-				MaxIdleConnsPerHost: 32, // seems about optimal, see #2805
-			},
-			DialTimeout: caddy.Duration(10 * time.Second),
-		}
+		t := &HTTPTransport{}
 		err := t.Provision(ctx)
 		if err != nil {
 			return fmt.Errorf("provisioning default transport: %v", err)