Benchmarking: Add JMH benchmark for ReplaySubject.

[daschl]

Adds some benches for the replay subject.

I.E.

Benchmark                                     (nextRuns)   Mode   Samples         Mean   Mean error    Units
r.s.ReplaySubjectPerf.subscribeAfterEvents             1  thrpt         5  1898239.827    57634.047    ops/s
r.s.ReplaySubjectPerf.subscribeAfterEvents          1024  thrpt         5    35680.927      470.929    ops/s
r.s.ReplaySubjectPerf.subscribeAfterEvents       1048576  thrpt         5       32.427        1.462    ops/s
r.s.ReplaySubjectPerf.subscribeBeforeEvents            1  thrpt         5  1427820.733    19194.452    ops/s
r.s.ReplaySubjectPerf.subscribeBeforeEvents         1024  thrpt         5    36835.800      264.748    ops/s
r.s.ReplaySubjectPerf.subscribeBeforeEvents      1048576  thrpt         5       35.520        1.145    ops/s

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[benjchristensen]

Nice, thank you!