Standalone script/binary to generate mcts search log json files

[shindavid]

We currently have a script, courtesy of @lichensongs, to visualize the MCTS search process in a web-browser. The script works by loading a JSON search log, which can be generated by configuring an mcts::Manager appropriately.

Currently, the only spot where we do such a configuration is in cpp/src/mcts/main/UnitTests.cpp (here). The context is for testing against a golden file for unit-testing purposes, using a mock neural-network evaluation service.

It would be generally useful to expose a way to generate such a search log outside of this context. Perhaps something like:

./py/dump_mcts_search_log.py --game <game> --model <model_file> --actions 1,3,3

This would create a blank game, apply the given actions, and then do an MCTS search on the resultant state using the passed-in model. See the test_search() function in cpp/src/mcts/main/UnitTests.cpp.

Notes:

• We probably want a new c++ binary for this purpose. If we match the existing structure of our cmake files, this would mean making repetitive boilerplate changes in each cpp/src/games/*/CMakeLists.txt.
• The current visualizer script (py/mcts_viz/web_viz.py) assumes that the json log file you want to visualize lives in sample_search_logs/mcts_tests/. This assumption becomes inappropriate once we add a tool like the above.