HPCC-10490 Show primary clusters for XRef

In multi-thor setups, the XREF cluster list should show
primary thor instances. In this fix, getPrimaryThorProcesses()
is added to IConstWUClusterInfo. The method picks primary
thor instance only if thor name matches the group name.
Using that method, WsDfuXRef service shows primary thor
instances in the XREF cluster list.

Signed-off-by: Kevin Wang kevin.wang@lexisnexis.com