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/miner.cpp

@@ -109,7 +109,7 @@ std::unique_ptr<CBlockTemplate> BlockAssembler::CreateNewBlock(const CScript& sc
     pblocktemplate->vTxSigOpsCost.push_back(-1); // updated at end
 
     LOCK2(cs_main, mempool.cs);
-    CBlockIndex* pindexPrev = chainActive.Tip();
+    CBlockIndex* pindexPrev = ::ChainActive().Tip();
     assert(pindexPrev != nullptr);
     nHeight = pindexPrev->nHeight + 1;