MINOR: Optimize RemoteLogManager#buildFilteredLeaderEpochMap

[majialoong]

Optimize RemoteLogManager#buildFilteredLeaderEpochMap .

In the original method, there is no need to count the epochs without messages（epochsWithNoMessages） first and then remove them. We can directly calculate and get the results.

Use JMH test :

@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
@State(Scope.Thread)
public class LeaderEpochMapBenchmark {

    @Param({"1000", "10000", "100000"})
    private int mapSize;

    NavigableMap<Integer, Long> testMap;

    @Setup
    public void setup() {
        testMap = new TreeMap<>();
        for (int i = 0; i < mapSize; i++) {
            /**
             * 0,0
             * 1,0
             * 2,1
             * 3,1
             * ...
             */
            testMap.put(i, (long) (i / 2));
        }
    }

    @Benchmark
    public NavigableMap<Integer, Long> testOriginal() {
        // before optimize (make method public)
        return RemoteLogManager.buildFilteredLeaderEpochMap(testMap);
    }

    @Benchmark
    public NavigableMap<Integer, Long> testOptimized() {
        // after optimize (make method public)
        return RemoteLogManager.buildFilteredLeaderEpochMap2(testMap);
    }
}

Command :
./jmh-benchmarks/jmh.sh LeaderEpochMapBenchmark

Result :

[majialoong]

Hello @chia7712 @kamalcph @frankvicky , If have chance, please take a look this RP, thanks !