Reduces overhead of RedisClusterNode.forEachSlot

[be-hase]

While investigating this issue (#2045), I discovered a minor performance issue.

Current RedisClusterNode.forEachSlot calls BitSet.length every time it loops.
The overhead here seems to be big. Please see the framegraph as follows.

We can reduce the overhead by calling BitSet.length only once.
When measured with JMH, you can see that the throughput(of RedisClusterNode.forEachSlot) is improved.

Benchmark                                            Mode  Cnt       Score      Error  Units
RedisClusterNodeBenchmark.originalRedisClusterNode  thrpt   25   62364.605 ± 1167.717  ops/s <-- Before
RedisClusterNodeBenchmark.updatedRedisClusterNode   thrpt   25  100597.253 ±  705.268  ops/s <-- After

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[mp911de]

Thanks. That makes a lot of sense.

[mp911de]

Thank you for your contribution. That's merged and backported now.

[be-hase]

thanks :)