HPCC-12450 Return start/stop clock time for graph

In this fix, the start/stop clock time for each ECL graph
are read from workunit statistics interface (kind=
'WhenGraphStarted' and kind='WhenGraphFinished') and
returned by WsWorkunits.WUinfo response.

Signed-off-by: wangkx <kevin.wang@lexisnexis.com>