HPCC-8758 - Stall in LOOKUP JOIN within LOOP

Regression introduced by HPCC-3326, was causing lookup join to
deadlock on restart, because the sender and receiver threads
were not being restarted correctly. But also the rhs was not
being clearedup in preparation for another run.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #3939 from jakesmith/lookupjoin-loop

HPCC-8758 - Stall in LOOKUP JOIN within LOOP

Reviewed-By: Gavin Halliday <gavin.halliday@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>