Use PriorityQueue instead of TreeMap in FirstPassGroupingCollector.

[vsop-479]

Description

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[vsop-479]

Benchmark:

Benchmark                                   Mode  Cnt   Score    Error   Units
PriorityQueueBenchmark.getTopNWithHeap     thrpt   15  ≈ 10⁻³           ops/us
PriorityQueueBenchmark.getTopNWithTreeMap  thrpt   15  ≈ 10⁻⁴           ops/us

[vsop-479]

I added a new benchmark (FirstPassGroupingBenchmark) to simulate FirstPassGroupingCollector 's implementation.
It seems use heap can gain a better performance:

Benchmark                                       Mode  Cnt  Score   Error   Units
FirstPassGroupingBenchmark.getTopNWithHeap     thrpt   15  0.010 ± 0.002  ops/us
FirstPassGroupingBenchmark.getTopNWithTreeMap  thrpt   15  0.005 ± 0.001  ops/us

[vsop-479]

Emm~, Use heap will remove(pop) element in orderedGroups, which will result in TestGrouping#testRandom fail, since this test case getTopGroups many times.