[CB-4567] fix issue: "Benchmarks" ->"AutoBench" crashed on iOS

The problem seems to be that a lot of autoreleased objects fill up the
memory waiting to be released. So a solution is to add autorelease pool
scope to collect autoreleased objects and release them sooner.

FYI:http://stackoverflow.com/questions/13396560/cant-allocate-region-mal
loc-error-when-running-millions-of-iterations-of-loop