typedef std::map<uint256, CCoins> to CCoinsMap

This makes it possible to switch to a more efficient map type
without changing all occurences manually.

Merges half of #4413.

src/txdb.h

@@ -37,7 +37,7 @@ public:
     bool HaveCoins(const uint256 &txid);
     uint256 GetBestBlock();
     bool SetBestBlock(const uint256 &hashBlock);
-    bool BatchWrite(const std::map<uint256, CCoins> &mapCoins, const uint256 &hashBlock);
+    bool BatchWrite(const CCoinsMap &mapCoins, const uint256 &hashBlock);
     bool GetStats(CCoinsStats &stats);
 };