Use const CCoinsView's at some places.

At some places where it is possible (e. g., CheckInputs), use a const
version of CCoinsView instead of a non-const one.

src/txmempool.h

@@ -85,7 +85,7 @@ public:
      * all inputs are in the mapNextTx array). If sanity-checking is turned off,
      * check does nothing.
      */
-    void check(CCoinsViewCache *pcoins) const;
+    void check(const CCoinsViewCache *pcoins) const;
     void setSanityCheck(bool _fSanityCheck) { fSanityCheck = _fSanityCheck; }
 
     bool addUnchecked(const uint256& hash, const CTxMemPoolEntry &entry);