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/fastcgi/fastcgi.go

@@ -64,7 +64,7 @@ type Transport struct {
 	// Extra environment variables.
 	EnvVars map[string]string `json:"env,omitempty"`
 
-	// The duration used to set a deadline when connecting to an upstream.
+	// The duration used to set a deadline when connecting to an upstream. Default: `3s`.
 	DialTimeout caddy.Duration `json:"dial_timeout,omitempty"`
 
 	// The duration used to set a deadline when reading from the FastCGI server.
@@ -88,13 +88,22 @@ func (Transport) CaddyModule() caddy.ModuleInfo {
 // Provision sets up t.
 func (t *Transport) Provision(ctx caddy.Context) error {
 	t.logger = ctx.Logger(t)
+
 	if t.Root == "" {
 		t.Root = "{http.vars.root}"
 	}
+
 	t.serverSoftware = "Caddy"
 	if mod := caddy.GoModule(); mod.Version != "" {
 		t.serverSoftware += "/" + mod.Version
 	}
+
+	// Set a relatively short default dial timeout.
+	// This is helpful to make load-balancer retries more speedy.
+	if t.DialTimeout == 0 {
+		t.DialTimeout = caddy.Duration(3 * time.Second)
+	}
+
 	return nil
 }