Stream perf tuning 2

Exploring iterator based transformer, in two steps. First only changing to Iterator, then adding melding of operators. Not covering everything, just an early indication.

These results can be compared with #15305
Running on MacBook Pro, 2,3 GHz Intel Core i7, with java 1.7.0_45
akka-bench-jmh > jmh:run -o result9-iterator-meld.txt -r 3 -i 5 -wi 3 -f 3 -prof gc,stack -jvmArgs "-Xms1024m -Xms1024m -XX:+UseCompressedOops" akka.stream.FlowMapBenchmark.*

baseline before Transform with Iterator

    Benchmark                                     (dispatcherPoolSize) (inputBufferSize) (numberOfMapOps)   Mode   Samples         Mean   Mean error    Units
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                16                1  thrpt        15     1334.722       45.950   ops/ms
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                16                3  thrpt        15     1183.999       35.057   ops/ms
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                16                5  thrpt        15     1032.527       44.215   ops/ms
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                32                1  thrpt        15     1387.664       37.349   ops/ms
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                32                3  thrpt        15     1306.621       31.856   ops/ms
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                32                5  thrpt        15     1107.189       40.113   ops/ms

258f653 Transform with Iterator

    Benchmark                                     (dispatcherPoolSize) (inputBufferSize) (numberOfMapOps)   Mode   Samples         Mean   Mean error    Units
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                16                1  thrpt        15     1673.165       79.986   ops/ms
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                16                3  thrpt        15     1355.013       31.576   ops/ms
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                16                5  thrpt        15     1114.652       31.779   ops/ms
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                32                1  thrpt        15     1751.866       43.758   ops/ms
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                32                3  thrpt        15     1404.107       27.075   ops/ms
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                32                5  thrpt        15     1181.734       61.098   ops/ms

c382935 meld transform iterators

    Benchmark                                     (dispatcherPoolSize) (inputBufferSize) (numberOfMapOps)   Mode   Samples         Mean   Mean error    Units
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                16                1  thrpt        15     1717.114       33.623   ops/ms
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                16                3  thrpt        15     1243.303       24.122   ops/ms
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                16                5  thrpt        15     1153.754        8.026   ops/ms
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                16               10  thrpt        15      980.477       13.273   ops/ms
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                16               15  thrpt        15      788.748       27.032   ops/ms
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                32                1  thrpt        15     1781.289       29.267   ops/ms
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                32                3  thrpt        15     1281.246       23.206   ops/ms
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                32                5  thrpt        15     1144.894       16.271   ops/ms
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                32               10  thrpt        15      920.991       14.080   ops/ms
    a.s.FlowMapBenchmark.flow_map_500k_elements                      4                32               15  thrpt        15      779.691       12.619   ops/ms