Reduce number of iterations in update_and_bulk_insert.js FSM workload

The update_and_bulk_insert.js repeatedly updates and inserts 100 documents, for 50 iterations. On some very slow platforms this amount of operations can end up being slow to replicate especially for secondaries syncing from another secondary. We should start by reducing the number of iterations in this workload, to say, 30 iterations, and see if this test still causes replication issues on slow machines.