asio's underlying timers have fairly loose behavior on cancel. Specifically, a cancelled callback can be invoked with or without the cancelled error code.

In the case of dtors that invoke callbacks, this means that we must have a shared pointer around with a lifetime longer than the actual ASIOTimer so that we can check if we've been cancelled (because we can't look at the original object).

In addition, we can call cancel/set multiple times, so we should keep a counter around so we know if the callback we're in should be invoked, even if the timer itself is currently active