Add DLS tests for more realistic scenarios

[gavin-norman-sociomantic]

There should be a set of DLS tests where real-world scenarios are covered - multiple writers, simultaneous read/writes, many combinations of iterations/writers, etc. In the current test suite there's no way, for example, to ignore the last chunk of the data (so it's forcing node to flush), there's no way to trigger fiber race conditions (as there's only one active fiber at the time), etc.