Multiple speed improvements revolving around topN with HLL

Change serializer / deserializer for HyperLogLog
* Changed DirectDruidClient's InputStream handling. Is now ~10% faster for data heavy queries (broker<-->historical), and has lower variance in execution speed.
* Changed HLL Collector's toByteStream() method to be better optimized for small values. Is notably faster for small result quantities which fall into the sparse HLL bucket codepath.
  * No change for dense HLL which just uses a direct bytestream of the underlying byte data.

TopNNumericResultBuilder semi-aggressive loop unrolling for metricVals

Benchmark for HLL for sparse packing (small HLL bucket population):
```
HyperLogLogSerdeBenchmarkTest.benchmarkToByteBuffer[0]: [measured 100000 out of 100100 rounds, threads: 1 (sequential)]
 round: 0.00 [+- 0.00], round.block: 0.00 [+- 0.00], round.gc: 0.00 [+- 0.00], GC.calls: 216, GC.time: 0.42, time.total: 15.96, time.warmup: 0.22, time.bench: 15.74
HyperLogLogSerdeBenchmarkTest.benchmarkToByteBuffer[1]: [measured 100000 out of 100100 rounds, threads: 1 (sequential)]
 round: 0.00 [+- 0.00], round.block: 0.00 [+- 0.00], round.gc: 0.00 [+- 0.00], GC.calls: 217, GC.time: 0.45, time.total: 13.87, time.warmup: 0.02, time.bench: 13.85
HyperLogLogSerdeBenchmarkTest.benchmarkToByteBuffer[2]: [measured 100000 out of 100100 rounds, threads: 1 (sequential)]
 round: 0.00 [+- 0.00], round.block: 0.00 [+- 0.00], round.gc: 0.00 [+- 0.00], GC.calls: 55, GC.time: 0.16, time.total: 4.13, time.warmup: 0.00, time.bench: 4.12
HyperLogLogSerdeBenchmarkTest.benchmarkToByteBuffer[3]: [measured 100000 out of 100100 rounds, threads: 1 (sequential)]
 round: 0.00 [+- 0.00], round.block: 0.00 [+- 0.00], round.gc: 0.00 [+- 0.00], GC.calls: 55, GC.time: 0.16, time.total: 4.30, time.warmup: 0.00, time.bench: 4.30
HyperLogLogSerdeBenchmarkTest.benchmarkToByteBuffer[4]: [measured 100000 out of 100100 rounds, threads: 1 (sequential)]
 round: 0.00 [+- 0.00], round.block: 0.00 [+- 0.00], round.gc: 0.00 [+- 0.00], GC.calls: 8, GC.time: 0.03, time.total: 1.10, time.warmup: 0.00, time.bench: 1.09
HyperLogLogSerdeBenchmarkTest.benchmarkToByteBuffer[5]: [measured 100000 out of 100100 rounds, threads: 1 (sequential)]
 round: 0.00 [+- 0.00], round.block: 0.00 [+- 0.00], round.gc: 0.00 [+- 0.00], GC.calls: 8, GC.time: 0.03, time.total: 0.72, time.warmup: 0.00, time.bench: 0.72
HyperLogLogSerdeBenchmarkTest.benchmarkToByteBuffer[6]: [measured 100000 out of 100100 rounds, threads: 1 (sequential)]
 round: 0.00 [+- 0.00], round.block: 0.00 [+- 0.00], round.gc: 0.00 [+- 0.00], GC.calls: 1, GC.time: 0.00, time.total: 0.60, time.warmup: 0.00, time.bench: 0.60
HyperLogLogSerdeBenchmarkTest.benchmarkToByteBuffer[7]: [measured 100000 out of 100100 rounds, threads: 1 (sequential)]
 round: 0.00 [+- 0.00], round.block: 0.00 [+- 0.00], round.gc: 0.00 [+- 0.00], GC.calls: 2, GC.time: 0.01, time.total: 0.26, time.warmup: 0.00, time.bench: 0.25
```
let's extract the conversion to sparse as a static function and make the protected variables private again.