1. Support the new external data extension in ONNX 1.4 onnx/onnx#678
2. Enable onnxruntime_perf_test in Mac Build
3. move path_lib.h from onnx_test_runner source dir to onnxruntime_framework
4. Enable memory planner for string tensors
5. Make memory planner always enabled, to simplify model loading logic
6. Delete some duplicated code between onnxruntime_perf_test and onnx_test_runner
7. Delete win_getopt_mb lib.
8. Remove the dependency on Pathcch lib, which is only available on Windows 8 and newer.
