Allow multithreading to `OperationQueue` with a feature

[babolivier]

We currently cannot require a trait bound on Send for queue items because of some compatibility issues with Thunderbird's internal types, but having it would be helpful with runtimes like tokio that make spawning a local task a bit painful (don't refer to the example in the readme, it's a bit incorrect in that regard). We could introduce a send feature that redefines QueuedOperation with such a bound, which would allow the queue to be used more easily with runtimes that are friendlier to use in this kind of context.