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/node/interfaces.cpp

@@ -447,7 +447,7 @@ public:
     {
         LOCK(cs_main);
         const CChain& active = Assert(m_node.chainman)->ActiveChain();
-        if (CBlockIndex* fork = FindForkInGlobalIndex(active, locator)) {
+        if (CBlockIndex* fork = g_chainman.m_blockman.FindForkInGlobalIndex(active, locator)) {
             return fork->nHeight;
         }
         return nullopt;