Multithreading support

[Matteomt]

Hi!

I'm facing some strange issue with the @backoff.on_exception decorator apparently being ineffective (as if it was not present, or maybe always hitting max_time before I expected it) in a situation where I'm trying to use it with multiple threads.
Specifically, I have a thread pool running multiple concurrent requests to a rate limited service.

I looked for threading or concurrency support in the readme, the open and closed issues, and the discussions here.

Question: is the decorator thread-safe?