scripted-diff: replace chainActive -> ::ChainActive()

Though at the moment ChainActive() simply references `g_chainstate.m_chain`,
doing this change now clears the way for multiple chainstate usage and allows
us to script the diff.

-BEGIN VERIFY SCRIPT-
git grep -l "chainActive" | grep -E '(h|cpp)$' | xargs sed -i '/chainActive =/b; /extern CChain& chainActive/b; s/\(::\)\{0,1\}chainActive/::ChainActive()/g'
-END VERIFY SCRIPT-

src/index/base.cpp

@@ -63,9 +63,9 @@ bool BaseIndex::Init()
     if (locator.IsNull()) {
         m_best_block_index = nullptr;
     } else {
-        m_best_block_index = FindForkInGlobalIndex(chainActive, locator);
+        m_best_block_index = FindForkInGlobalIndex(::ChainActive(), locator);
     }
-    m_synced = m_best_block_index.load() == chainActive.Tip();
+    m_synced = m_best_block_index.load() == ::ChainActive().Tip();
     return true;
 }
 
@@ -74,15 +74,15 @@ static const CBlockIndex* NextSyncBlock(const CBlockIndex* pindex_prev) EXCLUSIV
     AssertLockHeld(cs_main);
 
     if (!pindex_prev) {
-        return chainActive.Genesis();
+        return ::ChainActive().Genesis();
     }
 
-    const CBlockIndex* pindex = chainActive.Next(pindex_prev);
+    const CBlockIndex* pindex = ::ChainActive().Next(pindex_prev);
     if (pindex) {
         return pindex;
     }
 
-    return chainActive.Next(chainActive.FindFork(pindex_prev));
+    return ::ChainActive().Next(::ChainActive().FindFork(pindex_prev));
 }
 
 void BaseIndex::ThreadSync()
@@ -168,7 +168,7 @@ bool BaseIndex::Commit()
 bool BaseIndex::CommitInternal(CDBBatch& batch)
 {
     LOCK(cs_main);
-    GetDB().WriteBestBlock(batch, chainActive.GetLocator(m_best_block_index));
+    GetDB().WriteBestBlock(batch, ::ChainActive().GetLocator(m_best_block_index));
     return true;
 }
 
@@ -280,9 +280,9 @@ bool BaseIndex::BlockUntilSyncedToCurrentChain()
 
     {
         // Skip the queue-draining stuff if we know we're caught up with
-        // chainActive.Tip().
+        // ::ChainActive().Tip().
         LOCK(cs_main);
-        const CBlockIndex* chain_tip = chainActive.Tip();
+        const CBlockIndex* chain_tip = ::ChainActive().Tip();
         const CBlockIndex* best_block_index = m_best_block_index.load();
         if (best_block_index->GetAncestor(chain_tip->nHeight) == chain_tip) {
             return true;

src/index/txindex.cpp

@@ -236,7 +236,7 @@ bool TxIndex::Init()
     // Attempt to migrate txindex from the old database to the new one. Even if
     // chain_tip is null, the node could be reindexing and we still want to
     // delete txindex records in the old database.
-    if (!m_db->MigrateData(*pblocktree, chainActive.GetLocator())) {
+    if (!m_db->MigrateData(*pblocktree, ::ChainActive().GetLocator())) {
         return false;
     }