Add relcache invaladation synchronization.

[wenchaozhang-123]

authored-by: zhangwenchao zwcpostgres@gmail.com

closes: #ISSUE_Number

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[avamingli]

The title Add relcache invaladation synchronization sounds like a feature, but we just move codes to public with some hooks.

And to make others clear, could you please add some messages to tell the reason and what we do.