Tracers whose settings change start over on reload

A cluster rehearsal moved a Log tracer to another directory: the write
was reported x:settingsReload applied:true, but the tracer kept writing
to the old file until a restart. Telemetry::update only refreshed each
running tracer's events, level and lossiness; a tracer's own settings
(path, prefix, rotation, format, endpoint, headers, ...) stayed as built.

Each tracer now carries a hash of the registry object it was built
from, less the fields that change in place. The reload compares it with
the running tracer's: unchanged ones are updated in place as before,
changed ones are started over, new ones started and removed ones
stopped. Only tracers this server started are removed; upstream removed
every subscriber not in the settings, which also cut off live-tracing
streams on each reload.

Starting over is a swap in the collector, so no event is lost or
written twice: a subscriber registered under a running one's id
replaces it between two collection passes. The old one's batch is sent
first (what its full channel can't take moves to the new one), and
dropping it closes its channel, so its task writes what is queued and
ends. Per tracer kind:

- Log: a tracer started over on the same files (rotation or format
  changed) waits for the old one to finish, so lines don't interleave.
- Webhook: the task held a sender of its own channel for retries, so
  it never ended; retries now use a weak sender, and pending events are
  posted when the channel closes.
- OpenTelemetry: pending logs and spans are exported when the channel
  closes instead of dropped, and a span that was open across the swap
  is exported by the new tracer with the events it saw.
- Console and journal: nothing kept between batches.
- Trace history: built from the tracing store, which takes a restart,
  so it is never started over.

No kind needs a restart, so x:settingsReload doesn't gain one.

system::tracer_reload::tracer_reload_tests (new): a Log tracer created
over JMAP writes to its directory; its path is changed over JMAP while
2000 numbered events are emitted; after the reload, events land in the
new file and not the old one, each numbered event is in exactly one of
the two files, and a destroyed tracer writes nothing. On main the new
file never appears.

tests/src/system/mod.rs

@@ -24,6 +24,7 @@ pub mod quota;
 pub mod reload; // inbuxa: reloads and build errors
 pub mod security;
 pub mod task;
+pub mod tracer_reload; // inbuxa: tracers start over when their settings change
 pub mod tenant;
 pub mod undelete;