Fixing errors when API call to update post restrictions is called multiple times

[dparker1005]

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Changes proposed in this Pull Request:

Previously, dispatch( 'pmpro/require-membership' ).saveRestrictedLevels(); would be called multiple times during a single post save and the API could have race conditions because of it. This ensures that the API is only called once per post save.

Fix is based on this GitHub issue comment until there is a better solution for this in core WP:
WordPress/gutenberg#17632 (comment)

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