Add context for TODO "Better support for concurrency"

[alangibson]

Please add some context around the TODO item "Better support for concurrency." What exactly are the current weaknesses when it comes to concurrency?

[tylerflint]

@sdomino, @glinton Maybe you guys have a better sense of this?

[glinton]

Just off the cuff guessing, I'd say it has something to do with file locking. I can't remember to what extent it is implemented, but iirc there is some file locking. Running blame on the readme shows that todo item was written late 2015 and I think there has been some refactoring since then so it may be irrelevant (seeing that there are some rw locks). @sdomino would definitely be a better source though.