struct async_sequence_id

A dispatcher-specific sequence identifier, which identifies a set of actions

with a total ordering of execution: each subsequent action will always

observe side-effects from previous actions, if the thread(s) performing those

actions have the same sequence identifier.

For example, a dispatcher backed by a thread pool may choose to implement

sequences by acquiring a sequence-specific lock before running any actions

from that sequence, ensuring mutual exclusion within each sequence.