HPCC-8594 - Fix splitter activity timings + missing indexread timing

timeMinMs/timeMaxMs were being accumulated based on an uninitialized
member variable, producing spurious results for splitter timings
and effecting the activity immediately downstream from the splitter.

Also, Indexread was accounting for activity timing in nextRow()

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #3880 from jakesmith/hpcc-8594

HPCC-8594 - Fix splitter activity timings + missing indexread timing

Reviewed-By: Gavin Halliday <gavin.halliday@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>