Problem

close #10049, close #10030, close #8861

part of #9114

The legacy gc process calls get_latest_gc_cutoff, which uses a Rcu different than the gc_info struct. In the gc_compaction_smoke test case, the "latest" cutoff could be lower than the gc_info struct, causing gc-compaction to collect data that could be accessed by latest_gc_cutoff. Technically speaking, there's nothing wrong with gc-compaction using gc_info without considering latest_gc_cutoff, because gc_info is the source of truth. But anyways, let's fix it.

Summary of changes

• gc-compaction uses latest_gc_cutoff instead of gc_info to determine the gc horizon.
• if a gc-compaction is scheduled via tenant compaction iteration, it will take the gc_block lock to avoid racing with functionalities like detach ancestor (if it's triggered via manual compaction API without scheduling, then it won't take the lock)