common/obj_bencher.cc: faster object name generation

common/obj_bencher.cc: faster object name generation

generate_object_name() uses std::ostringstream to build object name
and calls generate_object_prefix() (which uses another std::ostringstream),
making it quite inefficient and actually being one of limiting factors
in small-object benchmarks. New version that does everything on one char
array and within one function is up to 3,8x faster (in my testing bumping
throughput from 35MB/s to 40MB/s on rados bench rand -t 128 on 1KB objects).

Signed-off-by: Piotr Dałek <piotr.dalek@ts.fujitsu.com>