Flush stream after benchmarkStarting in ConsoleReporter

This means that the user will see the estimation of full benchmark
running time when it is available, unlike now when it often only
ends up flushed after the benchmark is fully finished.

This means that the user will almost immediately see the start
of table like this

```
benchmark name                       samples       iterations    estimated
                                     mean          low mean      high mean
                                     std dev       low std dev   high std dev
-------------------------------------------------------------------------------
Fill vector generated                          100            54     3.0834 ms
```

This presents significant improvement in user experience especially
for long running benchmarks.

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