Add index CR UT for basic merge

[koujl]

https://docs.google.com/presentation/d/1NUvu1ZlDKqJWfBayg2lSWHSB6TEaZ4Y3VbnGc996drM/edit#slide=id.g2f8c68a8252_1_10

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Add basic index CR logic & UT.

Major changes including:

• Add freed nodes to the dependency graph for recovery
• Reconstruct our handling strategy in wb_cache::recover() for recovered new/freed buffers
• Add protection for several corner cases in index_table::repair_link()
• Add flip point for freed nodes

As well as some bug fixes:

• Fix several down_buffer counter leaks
• Fix a bug in wb_cache async_flush that may cause one buffer get processed multiple times
• Fix some bugs in racing conditions during homestore restart

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There are still some known issues in some corner cases, such as incorrect next_node pointer. However, since solving these may introduce complicated modification to the current logic, let's leave them to future PRs and discuss later.