Performance: AM scaleability is 10% slower in 2.4 compared to 0.23.9

Performance comparison benchmarks from 2.x against 0.23 shows AM scalability benchmark's runtime is approximately 10% slower in 2.4.0. The trend is consistent across later releases in both lines, latest release numbers are:

2.4.0.0 runtime 255.6 seconds (avg 5 passes)
0.23.9.12 runtime 230.4 seconds (avg 5 passes)
Diff: -9.9%

AM Scalability test is essentially a sleep job that measures time to launch and complete a large number of mappers.

The diff is consistent and has been reproduced in both a larger (350 node, 100,000 mappers) perf environment, as well as a small (10 node, 2,900 mappers) demo cluster.