TaskGroup provides a mechanism for tracking execution of multiple
concurrent tasks and receiving notification when all tasks have
completed. This is useful for coordinating asynchronous operations in
the ORC runtime.
TaskGroup::Token is an RAII handle representing participation in a
group. The group cannot complete while any valid (non-default) Token
exists.
TaskGroup::addOnComplete registers callbacks to run when the group
closes and all tokens are released. (Callbacks registered after
completion run immediately).
TalkGroup::close seals the group: no new tokens can be acquired after
close is called.
All methods may be called concurrently from multiple threads.
