HPCC-12250 Allow pluggable interface to workunit creation

Initial split of CWorkUnitFactory - just moving code (and a bit of renaming)
without yet getting the dali code out of the base class.

Also deleted some unused functions and rationalized slightly.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
HPCC-12250 Allow pluggable interface to workunit creation

More complete split of CWorkUnitFactory - should be ready for writing a
Cassandra version of the factory / workunit classes. Seill need to think
about how locking is done.

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