HPCC-11494 Roxie dali notification issues

Backport some changes from candidate-5.0.0 made in connection with HPCC-11253,
to avoid multiple reloads on parallel threads when notifications received fro
packagemap updates.

Also contains the fixes for HPCC-11293 (delayed unload of slave queries).

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>