distbench_engine.cc: Fix race conditions in shutdown. Add missing calls to shutdown methods of the protocol driver. Fix computation of pending_iterations when about to start a new one.
distbench_engine.cc: Fix race conditions in shutdown.
Add missing calls to shutdown methods of the protocol driver.
Fix computation of pending_iterations when about to start a new one.

google/distbench,google/distbench,google/distbench