refactor: pcoinsTip -> CChainState::CoinsTip()

This aliasing makes subsequent commits easier to review; eventually CoinsTip()
will return the CCoinsViewCache managed by CChainState.

src/interfaces/node.cpp

@@ -232,7 +232,7 @@ public:
     bool getUnspentOutput(const COutPoint& output, Coin& coin) override
     {
         LOCK(::cs_main);
-        return ::pcoinsTip->GetCoin(output, coin);
+        return ::ChainstateActive().CoinsTip().GetCoin(output, coin);
     }
     std::string getWalletDir() override
     {