[FLINK-4385] [table] Union on Timestamp fields does not work

[wuchong]

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This PR fix the problem mentioned in JIRA. The reason is that it will call estimateRowSize and encounter type mismatch for temporal types in batch mode.

Another fix is the IntervalTypeInfo's Comparator instantiation, the constructor of TypeComparator should be boolean.

[twalthr]

@wuchong Thanks for the fix! Will merge...