HPCC-12220 SuperHashTable kill not resetting tablecount

releaseAll() was not resetting tablecount, as a consequence
the HT size would spuriously continue to grow + increase the
cost of releaseAll's time to walk the HT to clear elements

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #6436 from jakesmith/hpcc-12220

HPCC-12220 SuperHashTable kill not resetting tablecount

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>