refactor: pcoinsTip -> CChainState::CoinsTip()

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

src/validation.h

@@ -505,6 +505,9 @@ public:
         CBlockIndex** ppindex) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 };
 
+/** Global variable that points to the active CCoinsView (protected by cs_main) */
+extern std::unique_ptr<CCoinsViewCache> pcoinsTip;
+
 /**
  * CChainState stores and provides an API to update our local knowledge of the
  * current best chain.
@@ -566,6 +569,12 @@ public:
      */
     std::set<CBlockIndex*, CBlockIndexWorkComparator> setBlockIndexCandidates;
 
+    //! @returns A reference to the in-memory cache of the UTXO set.
+    CCoinsViewCache& CoinsTip()
+    {
+        return *::pcoinsTip;
+    }
+
     /**
      * Update the on-disk chain state.
      * The caches and indexes are flushed depending on the mode we're called with
@@ -662,9 +671,6 @@ BlockMap& BlockIndex();
 /** Global variable that points to the coins database (protected by cs_main) */
 extern std::unique_ptr<CCoinsViewDB> pcoinsdbview;
 
-/** Global variable that points to the active CCoinsView (protected by cs_main) */
-extern std::unique_ptr<CCoinsViewCache> pcoinsTip;
-
 /** Global variable that points to the active block tree (protected by cs_main) */
 extern std::unique_ptr<CBlockTreeDB> pblocktree;