Use ZeroLog.LogManager.Flush() in benchmarks

[ltrzesniewski]

This PR updates ZeroLog in benchmarks and replaces some usages of its LogManager.Shutdown method with LogManager.Flush.

When I sent #127, I made a note that we should expose a Flush method for benchmarking, as I had to use Shutdown, which worked but was incorrect (it allocates, takes a longer time, does more than necessary, etc). We were using an internal WaitUntilQueueIsEmpty method in ZeroLog benchmarks but I couldn't use it here. Oops. 😅

This PR therefore uses the new Flush method where it is the correct thing to do. Benchmark results are barely affected though.

[neuecc]

That's great.
I'd like to set up a fair benchmark as accurately as possible, so this is really helpful!