Adjust the bigtable fill rates in order to dial down MP writes at start of
txnid2 benchmark.
  - Partitioned big table fills at twice previous rate
  - Replicated big table fills at ~10% of previous rate
txnid2: add ratelimit control
txnid2 run.sh: clean all catalogs (minor)
txnid2: fix bug: at end of benchmark worker threads may hang in shutdown because ratelimit permits are no longer being released