Move block-storage-related logic to ChainstateManager

Separate the notion of which blocks are stored on disk, and what data is in our
block index, from what tip a chainstate might be able to get to. We can use
chainstate-agnostic data to determine when to store a block on disk (primarily,
an anti-DoS set of criteria) and let the chainstates figure out for themselves
when a block is of interest for being a candidate tip.

Note: some of the invariants in CheckBlockIndex are modified, but more work is
needed (ie to move CheckBlockIndex to ChainstateManager, as most of what
CheckBlockIndex is doing is checking the consistency of the block index, which
is outside of Chainstate).

src/bench/load_external.cpp

@@ -49,14 +49,13 @@ static void LoadExternalBlockFile(benchmark::Bench& bench)
         fclose(file);
     }
 
-    Chainstate& chainstate{testing_setup->m_node.chainman->ActiveChainstate()};
     std::multimap<uint256, FlatFilePos> blocks_with_unknown_parent;
     FlatFilePos pos;
     bench.run([&] {
         // "rb" is "binary, O_RDONLY", positioned to the start of the file.
         // The file will be closed by LoadExternalBlockFile().
         FILE* file{fsbridge::fopen(blkfile, "rb")};
-        chainstate.LoadExternalBlockFile(file, &pos, &blocks_with_unknown_parent);
+        testing_setup->m_node.chainman->LoadExternalBlockFile(file, &pos, &blocks_with_unknown_parent);
     });
     fs::remove(blkfile);
 }