There's now double bookkeeping:
- All delayed calls are in a map whose key is the next
invocation time. Since it's a map (and not an unordered_map)
it's sorted just the way we want to have it.
- In addition, there's an unordered set for each tag.
With this arrangement we can easily invoke the delayed calls
in the right order and be able to efficiently remove all
delayed calls related to a particular tag.
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