HPCC-9828 - Resolve package watcher notification/reload deadlock

Multiple CDaliPackageWatcher's could be notified in parallel
on different threads. Each invoked a reload of all packages in
the manager, which could result in deadlock, if two threads
both tried to unregister a watcher that was already in
it's notify method.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #4734 from jakesmith/hpcc-9828

HPCC-9828 - Resolve package watcher notification/reload deadlock

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>