This PR fixes a few issues that occurred on high nested task load (TestNestedTask.test_multiple_recursive_task):

1. AsyncPriorityQueue now correctly removes tasks with a negative priority value;
2. the worker VanillaHeartbeatManager now watches all processors (not only the active one), and better handle zombie and dead processes;
3. prevents the processor to be suspended while still waiting on the pausing event (caused an unsupported re-entrant call on Linux);
4. the processor manager now handles correctly balance/cancel and suspension commands on a processor that is not yet initialized.