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ARROW-3248: [C++] Add "arrow" prefix to Arrow core unit tests, use PREFIX instead of file name for csv, io, ipc tests. Modular target cleanup

I added a section to the cpp/README.md about the modular build targets.

Author: Wes McKinney <wesm+git@apache.org>

Closes #3152 from wesm/ARROW-3248 and squashes the following commits:

ba3a3e58c <Wes McKinney> Need to add arrow- prefix to some Travis scripts
1f3daaf78 <Wes McKinney> Rename io/ipc tests/executables. Add appropriate labels/prefixes to all unit tests/benchmarks. Add labels option to ADD_BENCHMARK