[c++11] Use std::unique_ptr for block creation.

CreateNewBlock returns a pointer for which the caller takes ownership.
Use std::unique_ptr to make this explicit and simplify handling of these
objects in getblocktemplate.

src/miner.h

@@ -160,7 +160,7 @@ private:
 public:
     BlockAssembler(const CChainParams& chainparams);
     /** Construct a new block template with coinbase to scriptPubKeyIn */
-    CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn);
+    std::unique_ptr<CBlockTemplate> CreateNewBlock(const CScript& scriptPubKeyIn);
 
 private:
     // utility functions