validation: Move FindForkInGlobalIndex to BlockManager

[META] This commit should be followed up by removing the comments and
       assertions meant only to show that the change is correct.

FindForkInGlobalIndex only acts on BlockManager.

Note to reviewers: Since FindForkInGlobalIndex is always called with
::ChainActive() as its first parameter, it is possible to move
FindForkInGlobalIndex to CChainState and remove this const CChain&
parameter to instead use m_chain. However, it seems like the original
intention was for FindForkInGlobalIndex to work with _any_ chain, not
just the current active chain. Let me know if this should be changed.

src/index/base.cpp

@@ -62,7 +62,7 @@ bool BaseIndex::Init()
     if (locator.IsNull()) {
         m_best_block_index = nullptr;
     } else {
-        m_best_block_index = FindForkInGlobalIndex(::ChainActive(), locator);
+        m_best_block_index = g_chainman.m_blockman.FindForkInGlobalIndex(::ChainActive(), locator);
     }
     m_synced = m_best_block_index.load() == ::ChainActive().Tip();
     return true;