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/caddyfile.go

@@ -806,7 +806,9 @@ func (h *Handler) FinalizeUnmarshalCaddyfile(helper httpcaddyfile.Helper) error
 //         tls_trusted_ca_certs <cert_files...>
 //         tls_server_name <sni>
 //         keepalive [off|<duration>]
+//         keepalive_interval <interval>
 //         keepalive_idle_conns <max_count>
+//         keepalive_idle_conns_per_host <count>
 //         versions <versions...>
 //         compression off
 //         max_conns_per_host <count>
@@ -966,6 +968,19 @@ func (h *HTTPTransport) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 				}
 				h.KeepAlive.IdleConnTimeout = caddy.Duration(dur)
 
+			case "keepalive_interval":
+				if !d.NextArg() {
+					return d.ArgErr()
+				}
+				dur, err := caddy.ParseDuration(d.Val())
+				if err != nil {
+					return d.Errf("bad interval value '%s': %v", d.Val(), err)
+				}
+				if h.KeepAlive == nil {
+					h.KeepAlive = new(KeepAlive)
+				}
+				h.KeepAlive.ProbeInterval = caddy.Duration(dur)
+
 			case "keepalive_idle_conns":
 				if !d.NextArg() {
 					return d.ArgErr()