Use caching variant of erc-parse-prefix internally

Use caching variant of erc-parse-prefix internally

* lisp/erc/erc-common.el (erc--parsed-prefix): New struct to help with
tasks that depends on the advertised "PREFIX" parameter.
* lisp/erc/erc.el (erc-parse-prefix): Rework slightly for readability.
(erc--parsed-prefix): New variable and function of the same name for
caching the reversed result of `erc-parse-prefix' locally per server.
(erc-channel-receive-names): Use value stored in `erc--parsed-prefix'.
* test/lisp/erc/erc-tests.el (erc-with-server-buffer): Only activate
spy around actual test case forms.
(erc--parse-prefix): New test.  (Bug#67220)