Resolve benchmark cleanup race condition in issue #20.

The multithreaded API for benchmarks provides that teardown can happen in
thread 0.  For this to be safe, all other threads executing the benchmark
function need to have exited.  Otherwise, thread 0 may begin to teardown shared
resources before the other threads have stopped using these resources as they
are in their last loop of while (KeepRunning()) { ... }.

This change creates a single exit point for KeepRunning() to return false.
When running a multithreaded benchmark, thread 0 blocks on KeepRunning() until
all other threads have exited.  This approach allows for there to be no change
to the user-facing API exemplified in the BM_MultiThreaded example.