common/async: spawn_throttle wraps call to asio::spawn()

common/async: spawn_throttle wraps call to asio::spawn()

cancellation of the parent must immediately cancel its children, which
only works if the children are on the same executor as the parent

prohibit child coroutines from being spawned on a different executor by
wrapping the call to asio::spawn() in a new spawn_throttle::spawn()
interface

expose an overload for asio::spawn()'s optional StackAllocator argument

Signed-off-by: Casey Bodley <cbodley@redhat.com>