Add mutex for write/close

[vyzo]

Closes #56

[Stebalien]

YOLO and I want to live.

[Stebalien]

@aarshkshah1992 we added this lock to prevent a concurrent write panic. However:

1. We should never be calling Write concurrently, as far as we know.
2. It should be safe to call Write and Close concurrently.

We'd still like to know why this was an issue because we're pretty sure this isn't the right fix. If you're bored, want to try looking into this? Don't spend too much time as multiple people have already sunk quite a bit of time trying to figure this out.