Simplifies the decode benchmarks to mirror the encode pattern more
closely: no per-iteration asm volatile barrier, no return-by-value of
APIBuffer through encode_message_for_proxy. CodSpeed callgrind has been
crashing inside Decode_ZWaveProxyFrame and the previous setup was the
main thing it had that the (passing) Encode_ZWaveProxyFrame did not.
Splitting these out from bench_proto_encode.cpp and bench_proto_decode.cpp
moves them to the end of the linker's static-init order. CodSpeed's
callgrind runner has been segfaulting immediately after measuring the
last existing decode benchmark (Decode_SwitchCommandRequest), and
isolating the new code into its own translation unit lets us see whether
the crash is triggered by one of the new benchmarks or by something
about the new USE_*_PROXY/USE_INFRARED/USE_RADIO_FREQUENCY defines
changing how api_pb2.cpp compiles.
J. Nick Koston