Replaced an os::sleep with Clock::settle in allocator benchmark.
Replaced an os::sleep with Clock::settle in allocator benchmark.

Currently, in HierarchicalAllocator_BENCHMARK_Test.AddAndUpdateSlave,
we are using 'sleep' to check if the operations are processed, this
is not accurate, we should use 'Clock::settle' instead.

Review: https://reviews.apache.org/r/50115/

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