Fix unbounded event-task leak on the per-connection barrier

[henrikbjorn]

Problem

Librevox::Listener::Base dispatches every ESL event through a single
per-connection Async::Barrier (@event_barrier):

@event_barrier.async do
  on_event(response)
  invoke_event_hooks(response)
end

In async, Barrier#async appends the task to an internal list and a task is
removed only by #wait / #cancel. librevox calls @event_barrier.wait
exactly once — in #connection_closed (socket drop) — and the read loop never
drains it between messages.

So on a long-lived inbound connection subscribed to events, every event
received permanently retains a task node (plus the finished Async::Task it
references, plus a node in the barrier's @finished queue). On a busy
FreeSWITCH that is thousands of nodes/second, freed only when the socket drops —
a slow but unbounded memory leak. This is the cause of the ~8 GB-over-~2-days
growth we saw in the answering_machine daemon.

A Barrier is a scatter-gather join primitive — it retains its tasks on
purpose so a later wait can join them. It's the wrong tool for supervising an
unbounded, fire-and-forget event stream where wait only happens at shutdown.

Fix

Track in-flight event tasks in a Set instead. Each task adds itself on entry
and removes itself in its own ensure, so a finished task is never retained.
#connection_closed still drains genuinely in-flight hooks by waiting on a
snapshot of the set.

The per-event task itself is still required (a hook may issue a command and
block on its reply, which arrives via the same reader loop — running it inline
would deadlock; see test_on_event_with_api_does_not_block_handle_response).
Only the retention of completed tasks changes.

Test

Adds test_completed_event_tasks_are_not_retained: after 100 events the
listener holds zero in-flight tasks. Full suite green (121 runs, 0 failures).

Note

This is a minimal, targeted fix. The spike/em-style-simplification branch
reworks this dispatch path more broadly and may supersede it — happy to fold the
change into that direction instead.