Remove StringBuilder in Logback TimestampConverter

[rahuldeepattri]

Fix for #90

The benchmark done using JMH show performance improvement.

    @Benchmark
    public String benchmarkStringBuilder() {
        Instant now = Instant.now();
        long timestamp = now.getEpochSecond() * 1_000_000_000L + now.getNano();

        StringBuilder appendTo = new StringBuilder();
        appendTo.append(timestamp);
        return appendTo.toString();
    }

    @Benchmark
    public String benchmarkString() {
        Instant now = Instant.now();
        long timestamp = now.getEpochSecond() * 1_000_000_000L + now.getNano();

        return String.valueOf(timestamp);
    }

Result:

Benchmark                                                           Mode     Cnt   Score   Error   Units
Comparison.benchmarkString                                         thrpt     100   2.130 ± 0.052  ops/ms
Comparison.benchmarkStringBuilder                                  thrpt     100   1.801 ± 0.028  ops/ms
Comparison.benchmarkString                                          avgt     100   0.469 ± 0.011   ms/op
Comparison.benchmarkStringBuilder                                   avgt     100   0.862 ± 0.038   ms/op
Comparison.benchmarkString                                            ss     100   0.591 ± 0.095   ms/op
Comparison.benchmarkStringBuilder                                     ss     100   0.966 ± 0.118   ms/op

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[rahuldeepattri]

Hi @KarstenSchnitter, sorry my mistake. Fixed the indentation.