When a promise that still tracks its future is destroyed, but after
future::get() or future::then() was called, the promise thinks it was
destroyed before generating a value, and fails an assertion.
Fix be detaching the promise from the future as soon as get() or then()
remove the value.
Following std::async(), seastar::async(func) causes func to be executed
in a seastar thread, where it can block using future<>::get(). Whatever
func returns is converted to a future, and returned as async()s return
value.
