Always run erc-server-send-queue via timer

Always run erc-server-send-queue via timer

* lisp/erc/erc-backend.el (erc--server-send, erc-server-send): Convert
the latter into a wrapper that calls the former, a "new" internal
generic function, so that built-in modules can do things like prepend
tags to outgoing messages and send messages over other transports or
proxy protocols.  Extend the `no-penalty' parameter to mean ERC will
schedule an imminent send via a timer.  And always run the function
`erc-server-send-queue' on a timer.  (Bug#67031)