Make it possible to include basho_bench in another node

This is a change to make it possible to run basho_bench included in another node. Our use case is that we have a node acting as a client of the system under test, and we'd like to drive it with basho_bench without the extra complexity of running a separate basho_bench node.

The changes amount to killing only the application, not the entire node, on fatal errors. When basho_bench is run standalone, the behaviour should be unchanged. Also, make it possible to run the test more than once (don't crash if basho_bench is already loaded).