CDiff multiple source sites to same destination

[iuravic]

A work in progress PR to add CDiff support for multibranded sites, or other sites which need merging of two different source sites to one destination.

Description

Two parts to this PR:

• update postmeta with original ID to contain site hostname -- this is now complete at the time of submitting this WIP PR
• update the content-diff-migrate-live-content command to use postmeta instead of log files with migrated IDs, and distinguish between multiple sources -- not done yet

Testing

Detailed testing is necessary, and I would propose doing this scenario:

• set up 3 sites, two sources and one destination
• create posts on source sites:
  • some posts which have different/unique IDs among themselves
  • some posts which use identical IDs (e.g. post ID 100 on first site, and a different post on second site but also has ID 100)
  • have those posts use featured images, again some thumbnail images with same post (attachment) IDs and others with unique
  • run a first content diff import
    • check all the restults -- posts integrity, featured images, media library
  • continue towards a second content refresh by setting up the two source sites more
    • add more posts -- also some with new same and new unique IDs, and thumbnails)
    • modify some existing posts -- change the featured image on some
    • import posts and check all newly added and modified data, including existing post added in first round

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• confirmed that PHPCS has been run