[FIX] core: allow signal handlers in multiprocess workers

Because Python only runs signal handlers on the main thread, native
calls (IO or accept(2)) can delay these handlers running. This is
especially problematic when one such call is completely stuck and
we're trying to dump the stack to diagnose the issue.

By running the actual worker's processing in a sub-thread and leaving
the main thread sleeping, worker processes should always be able to
handle signals.

closes odoo/odoo#30688