Fix "Too many open files" errors when running on large repo on OSX

[rowillia]

It appears we're running out of file handles when running on large repo on OSX due to waiting until all files have been
processed to join/close the subprocesses.

This PR joins/closes them as they finish, resolving the issue.

$ python3 -m libcst.tool codemod {codemod_name} {direectory}
Calculating full-repo metadata...
Executing codemod...
Codemodding {file}
Traceback (most recent call last):
  File "/Users/rwilliams/src/go/src/github.com/lyft/python-lyft-ingest/venv/lib/python3.6/site-packages/libcst/codemod/_cli.py", line 253, in _parallel_exec_process_stub
OSError: [Errno 24] Too many open files: '{file}'

Failed to codemod {file}

19.80s 24% complete, 01m 02s estimated for 741 files to go...Traceback (most recent call last):
  File "/opt/lyft/brew/Cellar/python36/3.6.5_1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/opt/lyft/brew/Cellar/python36/3.6.5_1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/Users/rwilliams/src/go/src/github.com/lyft/python-lyft-ingest/venv/lib/python3.6/site-packages/libcst/tool.py", line 833, in <module>
    main(os.environ.get("LIBCST_TOOL_COMMAND_NAME", "libcst.tool"), sys.argv[1:])
  File "/Users/rwilliams/src/go/src/github.com/lyft/python-lyft-ingest/venv/lib/python3.6/site-packages/libcst/tool.py", line 828, in main
    return lookup.get(args.action or None, _invalid_command)(proc_name, command_args)
  File "/Users/rwilliams/src/go/src/github.com/lyft/python-lyft-ingest/venv/lib/python3.6/site-packages/libcst/tool.py", line 581, in _codemod_impl
    repo_root=config["repo_root"],
  File "/Users/rwilliams/src/go/src/github.com/lyft/python-lyft-ingest/venv/lib/python3.6/site-packages/libcst/codemod/_cli.py", line 720, in parallel_exec_transform_with_prettyprint
    process.start()
  File "/opt/lyft/brew/Cellar/python36/3.6.5_1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/multiprocessing/process.py", line 105, in start
    self._popen = self._Popen(self)
  File "/opt/lyft/brew/Cellar/python36/3.6.5_1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/multiprocessing/context.py", line 223, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "/opt/lyft/brew/Cellar/python36/3.6.5_1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/multiprocessing/context.py", line 277, in _Popen
    return Popen(process_obj)
  File "/opt/lyft/brew/Cellar/python36/3.6.5_1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/opt/lyft/brew/Cellar/python36/3.6.5_1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/multiprocessing/popen_fork.py", line 65, in _launch
    parent_r, child_w = os.pipe()
OSError: [Errno 24] Too many open files

Summary

Test Plan

[codecov-io]

Codecov Report

Merging #283 into master will decrease coverage by 0.03%.
The diff coverage is 8.33%.

@@            Coverage Diff             @@
##           master     #283      +/-   ##
==========================================
- Coverage   93.96%   93.92%   -0.04%     
==========================================
  Files         219      219              
  Lines       21245    21253       +8     
==========================================
  Hits        19962    19962              
- Misses       1283     1291       +8     

Impacted Files	Coverage Δ
libcst/codemod/_cli.py	20.53% <8.33%> (-0.65%)	⬇️

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[jimmylai]

LGTM
@rowillia , thanks for your fix!