TiCDC table span ResolvedTs advancing can be improved

[hicqu]

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Currently, TiCDC uses cdc/puller.Frontier to advance ResolvedTs. It's based on a heap with some other complex data structures like skiplist.

For a given number set, heap is good for such situation: after every time we update one number in the set, we need to calculate top(k) for them. However, For advancing ResolvedTs, we just need to get top(1) after a duration, and there could be million updates in the duration. So heap won't make helpful in this case, just iterate all numbers in the set is fine enough.