* 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
