Session group and client logging fixes

[balat]

Bug fixes found after the quality pass (#908). Each commit does one thing, builds, and has a CHANGES entry.

• Close the service and data sessions of a group separately. Each site has one group of groups, a dlist that limits the number of groups of browser sessions. Mod_sessiongroups.Serv and Data both add an entry to it for a group, but its finaliser did not know which one had added the entry and always closed the data sessions of the group and removed its group data. As a result:

  • closing the last service session of a group closed the data sessions of the group and removed its group data;
  • State.discard_services ~scope:(Session_group _)closed the data sessions of the group and left its service sessions open, soState.discard` with that scope did not close them;
  • evicting the service entry of a group, when a site has too many groups, did the same.

  The entries now carry the kind of their sessions (Common.group_of_groups_entry) and the finaliser closes the sessions of that kind. New unit tests (test/unit/test_sessiongroups.ml) check, on each level (tab sessions of a browser session, browser sessions of a group, groups of a site) and for both kinds of sessions, that closing a state, emptying or evicting a group, or lowering a limit closes exactly the states it holds, and never those of the other kind. Without the fix, the service cases on the group and site levels fail.

• State: list each session group once in get_session_group_list. A group with both kinds of sessions has two entries, so its name was returned twice.

• Document the levels of State.set_max_*_states_for_group_or_subnet. The scope gives the level of the states that are limited, in the state that contains them: tab sessions in the browser session, browser sessions in the group or subnet, and groups in the group of groups of the site (a limit shared by service and data states). The documentation spoke of the sessions of the current group for every scope. The CHANGES entry of an earlier fix of these functions (Fix latent bugs found during the pre-release review #907), which said that the limit reached the wrong group, is corrected: it only did nothing when the group had no state of the other kind.

• Lib (client): honour ~encode in Url.string_of_url_path. The client version ignored it and printed a warning; it now encodes each segment, like the server version.

• Lib (client): log the exceptions of Lwt.async with Logs. The client Lwt.async_exception_hook used console.error, bypassing Logs. It now uses Logs.err, followed by the JavaScript stack of the error attached to the exception, when there is one.

Testing

• dune build @check at every commit; dune build @runtest (32 tests) and dune build @fmt at the head.
• os_template built against this branch (before the test and documentation changes), driven in a headless browser with the wasm and the JavaScript clients: no reload loop, client-side navigation and history, RPC with session and persistent Eliom references, Comet between two browser contexts, the sign-up form. Every browser session there goes through the group of groups (subnet groups). No exception in the server log.