HPCC-11033 Load WU timer info in one call for WU Details

The existing ECL WU Details page takes a long time to load when a
WU has many timer items or helper files. The load time is improved
by this fix. 1. WUinfo code will use WU Statistics data about timers
if the Statistics data is available. 2. If the Statistics data is
not avaliable, an new interface IConstWUTimerIterator is implemented
to get the timer data in one call. For an ECL WU with 4635 timer
items, the time to read timer data is decreased from >100 seconds to
<0.1 second. 3. This fix also reduces the time of reading helper
file data by combining 2 file lookup calls into one call.

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