Quality pass follow-ups

[balat]

Small follow-ups to the quality pass (#908). Each commit does one thing and builds.

• Delete unused files: the two api.odocl files (leftovers of the ocamlbuild documentation setup) and the .gitignore pattern for them; src/ppx/ppx_eliom.ml and ppx_eliom.mli (a single comment each, in no dune stanza) and doc/ppx.indexdoc, which only referenced that module and is read by nothing (the PPX manual is doc/ppx-syntax.mld).
• Client: factor the handling of the load mutex: Client_core.with_load_mutex replaces the four copies of the lock, unlock and recover pattern in call_ocaml_service, set_template_content, set_content_local and set_content. The only difference is on a cancelled wait for the mutex, where set_content_local and set_content no longer end a debug timer they have not started.
• Client: use Lwt.async instead of Lwt.ignore_result at four sites. The change is not observable for the form handlers (already under Lwt.async) and the initial replaceState (always pending at module initialisation). For popstate, an immediate failure of revisit (the client-app checks, or a synchronous DOM failure in restore_history_dom) now goes to Lwt.async_exception_hook instead of escaping the popstate handler as an uncaught exception; there is no default action to keep. The link handler keeps Lwt.ignore_result, with a comment: when change_page_uri_a fails at once, the exception escapes Client_core.raw_a_handler, the default action is not prevented and the browser follows the link itself.
• Logging (one commit each, with a CHANGES entry):
  • the Comet client logs its connection and request failures as warnings instead of Logs.app;
  • Mod_sessadmin logs the update of expiry dates at info level instead of Logs.app;
  • Eliom_react logs under its own source, eliom:react, instead of eliom:comet;

Testing

• dune build @check at every commit; dune build @runtest and dune build @fmt at the head.
• os_template built against this branch (at bd451bb, before set_content was converted with the same transformation as set_content_local), driven in a headless browser with the wasm and the JavaScript clients: no reload loop; client-side navigation keeps the JavaScript state, with back, forward and reload; RPC with session and persistent Eliom references; Comet notifications between two browser contexts; the sign-up form (an Eliom post_form, which goes through change_page_post_form) is sent by XHR and keeps the JavaScript state. No exception in the server log.