Two small changes in derecho_bw_test.cpp
Two small changes in derecho_bw_test.cpp

Only node 0 writes the result. This make sure that there's only one
entry on a shared file system such as Stampede's. Also, programs call
exit(0) instead of managed_group->leave since the latter sometimes
gets stuck.

Derecho-Project/derecho-unified,Derecho-Project/derecho-unified,Derecho-Project/derecho-unified,Derecho-Project/derecho-unified,Derecho-Project/derecho-unified