Thread safety

[antoine-levitt]

#105 suggests that planning is thread-safe. I just had a memory error in my code that was caused by me calling ldiv!(x,p,y) on an existing plan (which calls FFTW's planner). Backtrace here: https://gist.github.com/antoine-levitt/18e80e5de607158f65ee4022967a8a6b. Is this expected? If yes, it should be documented somewhere what exactly is type-safe and what is not in the API.

[antoine-levitt]

Should mention: this is on julia 1.3.0, FFTW 1.2.0

[stevengj]

Maybe we need a lock around mutating the Plan object to cache the inverse plan in ldiv