Unmaintained Deadlock Avoidance by Xon 1.0.21

• Workaround to prevent race conditioning when following someone who is already followed.

• Support for Merge Posts.

• Fix merging/hard-deleting posts.
  

Hoist post-transaction behaviour out of nested transactions for:

• Like/Unlike
• Moving Posts.

Implements workarounds using "UPDATE FOR" to reduce deadlocks for:

• Deleting posts.
• Liking a post.

• On post/thread saving, push search updates till after the transaction.
  • Intended for where the search system is external to a MySQL transaction.

• Support for User DataWriter's _postSaveAfterTransaction() - does not impact stock XenForo but for my future add-ons.

• Ensure correctness of when _postSaveAfterTransaction is called.