src: fix race condition in `~NodeTraceBuffer`

Libuv does not guarantee that handles have their close
callbacks called in the order in which they were added
(and in fact, currently calls them in reverse order).

This patch ensures that the `flush_signal_` handle
is no longer in use (i.e. its close callback has already
been run) when we signal to the main thread that
`~NodeTraceBuffer` may be destroyed.

The same applies for `~NodeTraceWriter`.

Credit for debugging goes to Gireesh Punathil.

Fixes: https://github.com/nodejs/node/issues/25512


Co-authored-by: Gireesh Punathil <gpunathi@in.ibm.com>

PR-URL: https://github.com/nodejs/node/pull/25896


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>